Shrink ComputedVisibility (bevyengine#6305)

# Objective
`ComputedVisibility` could afford to be smaller/faster. Optimizing the size and performance of operations on the component will positively benefit almost all extraction systems.

This was listed as one of the potential pieces of future work for bevyengine#5310.

## Solution
Merge both internal booleans into a single `u8` bitflag field. Rely on bitmasks to evaluate local, hierarchical, and general visibility.

Pros:

 - `ComputedVisibility::is_visible` should be a single bitmask test instead of two.
 - `ComputedVisibility` is now only 1 byte. Should be able to fit 100% more per cache line when using dense iteration.

Cons:

 - Harder to read.
 - Setting individual values inside `ComputedVisiblity` require bitmask mutations. 

This should be a non-breaking change. No public API was changed. The only publicly visible effect is that `ComputedVisibility` is now 1 byte instead of 2.

crates/bevy_render/src/view/visibility/mod.rs

@@ -54,12 +54,19 @@ impl Visibility {
     }
 }
 
+bitflags::bitflags! {
+    #[derive(Reflect)]
+    struct ComputedVisibilityFlags: u8 {
+        const VISIBLE_IN_VIEW = 1 << 0;
+        const VISIBLE_IN_HIERARCHY = 1 << 1;
+    }
+}
+
 /// Algorithmically-computed indication of whether an entity is visible and should be extracted for rendering
 #[derive(Component, Clone, Reflect, Debug, Eq, PartialEq)]
 #[reflect(Component, Default)]
 pub struct ComputedVisibility {
-    is_visible_in_hierarchy: bool,
-    is_visible_in_view: bool,
+    flags: ComputedVisibilityFlags,
 }
 
 impl Default for ComputedVisibility {
@@ -71,8 +78,7 @@ impl Default for ComputedVisibility {
 impl ComputedVisibility {
     /// A [`ComputedVisibility`], set as invisible.
     pub const INVISIBLE: Self = ComputedVisibility {
-        is_visible_in_hierarchy: false,
-        is_visible_in_view: false,
+        flags: ComputedVisibilityFlags::empty(),
     };
 
     /// Whether this entity is visible to something this frame. This is true if and only if [`Self::is_visible_in_hierarchy`] and [`Self::is_visible_in_view`]
@@ -81,7 +87,7 @@ impl ComputedVisibility {
     /// [`CoreStage::Update`] stage will yield the value from the previous frame.
     #[inline]
     pub fn is_visible(&self) -> bool {
-        self.is_visible_in_hierarchy && self.is_visible_in_view
+        self.flags.bits == ComputedVisibilityFlags::all().bits
     }
 
     /// Whether this entity is visible in the entity hierarchy, which is determined by the [`Visibility`] component.
@@ -90,7 +96,8 @@ impl ComputedVisibility {
     /// [`VisibilitySystems::VisibilityPropagate`] system label.
     #[inline]
     pub fn is_visible_in_hierarchy(&self) -> bool {
-        self.is_visible_in_hierarchy
+        self.flags
+            .contains(ComputedVisibilityFlags::VISIBLE_IN_HIERARCHY)
     }
 
     /// Whether this entity is visible in _any_ view (Cameras, Lights, etc). Each entity type (and view type) should choose how to set this
@@ -102,7 +109,8 @@ impl ComputedVisibility {
     /// Other entities might just set this to `true` every frame.
     #[inline]
     pub fn is_visible_in_view(&self) -> bool {
-        self.is_visible_in_view
+        self.flags
+            .contains(ComputedVisibilityFlags::VISIBLE_IN_VIEW)
     }
 
     /// Sets `is_visible_in_view` to `true`. This is not reversible for a given frame, as it encodes whether or not this is visible in
@@ -111,7 +119,16 @@ impl ComputedVisibility {
     /// label. Don't call this unless you are defining a custom visibility system. For normal user-defined entity visibility, see [`Visibility`].
     #[inline]
     pub fn set_visible_in_view(&mut self) {
-        self.is_visible_in_view = true;
+        self.flags.insert(ComputedVisibilityFlags::VISIBLE_IN_VIEW);
+    }
+
+    #[inline]
+    fn reset(&mut self, visible_in_hierarchy: bool) {
+        self.flags = if visible_in_hierarchy {
+            ComputedVisibilityFlags::VISIBLE_IN_HIERARCHY
+        } else {
+            ComputedVisibilityFlags::empty()
+        };
     }
 }
 
@@ -280,13 +297,12 @@ fn visibility_propagate_system(
     children_query: Query<&Children, (With<Parent>, With<Visibility>, With<ComputedVisibility>)>,
 ) {
     for (children, visibility, mut computed_visibility, entity) in root_query.iter_mut() {
-        computed_visibility.is_visible_in_hierarchy = visibility.is_visible;
         // reset "view" visibility here ... if this entity should be drawn a future system should set this to true
-        computed_visibility.is_visible_in_view = false;
+        computed_visibility.reset(visibility.is_visible);
         if let Some(children) = children {
             for child in children.iter() {
                 let _ = propagate_recursive(
-                    computed_visibility.is_visible_in_hierarchy,
+                    computed_visibility.is_visible_in_hierarchy(),
                     &mut visibility_query,
                     &children_query,
                     *child,
@@ -313,10 +329,10 @@ fn propagate_recursive(
             child_parent.get(), expected_parent,
             "Malformed hierarchy. This probably means that your hierarchy has been improperly maintained, or contains a cycle"
         );
-        computed_visibility.is_visible_in_hierarchy = visibility.is_visible && parent_visible;
+        let visible_in_hierarchy = visibility.is_visible && parent_visible;
         // reset "view" visibility here ... if this entity should be drawn a future system should set this to true
-        computed_visibility.is_visible_in_view = false;
-        computed_visibility.is_visible_in_hierarchy
+        computed_visibility.reset(visible_in_hierarchy);
+        visible_in_hierarchy
     };
 
     for child in children_query.get(entity).map_err(drop)?.iter() {
@@ -390,7 +406,7 @@ pub fn check_visibility(
                     }
                 }
 
-                computed_visibility.is_visible_in_view = true;
+                computed_visibility.set_visible_in_view();
                 let cell = thread_queues.get_or_default();
                 let mut queue = cell.take();
                 queue.push(entity);
@@ -412,7 +428,7 @@ pub fn check_visibility(
                     return;
                 }
 
-                computed_visibility.is_visible_in_view = true;
+                computed_visibility.set_visible_in_view();
                 let cell = thread_queues.get_or_default();
                 let mut queue = cell.take();
                 queue.push(entity);
@@ -518,7 +534,7 @@ mod test {
                 .entity(e)
                 .get::<ComputedVisibility>()
                 .unwrap()
-                .is_visible_in_hierarchy
+                .is_visible_in_hierarchy()
         };
         assert!(
             !is_visible(root1),