Put font data into Arc to reduce memory consumption

[StarStarJ]

egui never accesses the FontDefinitions' member fields mutably, except in fonts_tweak_ui where it cloned the FontDefinitions object anyway.

This patch reduces system memory consumption for shared font definitions.
And also removes some overhead from copying (e.g. for the per pixel_per_points font atlas)

Also it allows to keep a copy of the font definitions outside of egui.

In my App that uses international fonts:
Before:

New:

Note: If Arc is not wanted, then it could ofc be abstracted away.

I know this is quite a breaking change API wise, but would like to hear your opinion.

[StarStarJ]

E.g. it keeps a copy in ContextImpl and copies it into the fonts per pixels_per_point:

egui/crates/egui/src/context.rs

Line 599 in 707cd03

self.font_definitions.clone(),

from:

egui/crates/egui/src/context.rs

Lines 587 to 601 in 707cd03

	let fonts = self
	.fonts
	.entry(pixels_per_point.into())
	.or_insert_with(|| {
	#[cfg(feature = "log")]
	log::trace!("Creating new Fonts for pixels_per_point={pixels_per_point}");
	is_new = true;
	crate::profile_scope!("Fonts::new");
	Fonts::new(
	pixels_per_point,
	max_texture_side,
	self.font_definitions.clone(),
	)
	});

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5276-pr_reduce_memory_usage_font_definitions
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

Let's wait until after #5228 is merged

[emilk]

I think a simpler and smaller change would be

@@ -243,7 +243,7 @@ pub struct FontDefinitions {
     /// List of font names and their definitions.
     ///
     /// `epaint` has built-in-default for these, but you can override them if you like.
-    pub font_data: BTreeMap<String, FontData>,
+    pub font_data: BTreeMap<String, Arc<FontData>>,

[StarStarJ]

I think a simpler and smaller change would be

@@ -243,7 +243,7 @@ pub struct FontDefinitions {
     /// List of font names and their definitions.
     ///
     /// `epaint` has built-in-default for these, but you can override them if you like.
-    pub font_data: BTreeMap<String, FontData>,
+    pub font_data: BTreeMap<String, Arc<FontData>>,

Sounds good aswell

[emilk]

Much better!