Fix multiple partial updates of the same texture (#1338)

Co-authored-by: Wanja Zaeske <wanja.zaeske@dlr.de>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

epaint/src/textures.rs

@@ -39,7 +39,7 @@ impl TextureManager {
             filter,
         });
 
-        self.delta.set.insert(id, ImageDelta::full(image, filter));
+        self.delta.set.push((id, ImageDelta::full(image, filter)));
         id
     }
 
@@ -57,9 +57,10 @@ impl TextureManager {
                 // whole update
                 meta.size = delta.image.size();
                 meta.bytes_per_pixel = delta.image.bytes_per_pixel();
+                // since we update the whole image, we can discard all old enqueued deltas
+                self.delta.set.retain(|(x, _)| x != &id);
             }
-
-            self.delta.set.insert(id, delta);
+            self.delta.set.push((id, delta));
         } else {
             crate::epaint_assert!(false, "Tried setting texture {id:?} which is not allocated");
         }
@@ -175,7 +176,7 @@ impl TextureMeta {
 #[must_use = "The painter must take care of this"]
 pub struct TexturesDelta {
     /// New or changed textures. Apply before painting.
-    pub set: AHashMap<TextureId, ImageDelta>,
+    pub set: Vec<(TextureId, ImageDelta)>,
 
     /// Textures to free after painting.
     pub free: Vec<TextureId>,