Prevent accidentally dropping TexturesDelta (#8356)

We had a ton of issues around `TexturesDelta` that weren't properly
applied because we early-out of some function:
* https://github.com/emilk/egui/pull/8313
* https://github.com/emilk/egui/pull/8250
* https://github.com/emilk/egui/pull/8279

This PR changes texture updates, so that we always store them after
taking them out of `FullOutput` and keep the delta around until it's
actually applied (by passing &mut refs and draining instead of
iterating). So even if we add a new early return somewhere, that can't
break texture updates.

It also optimizes `TexturesDelta::append` by dropping any previous
deltas if there's a new `whole` delta or a `free`.

It also adds a debug assert that any `TexturesDelta` is empty when
dropped, as an additional safeguard in case the bug sneaks back in.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/eframe/src/web/app_runner.rs

@@ -324,7 +324,6 @@ impl AppRunner {
 
     /// Paint the results of the last call to [`Self::logic`].
     pub fn paint(&mut self) {
-        let textures_delta = std::mem::take(&mut self.textures_delta);
         let clipped_primitives = std::mem::take(&mut self.clipped_primitives);
 
         if let Some(clipped_primitives) = clipped_primitives {
@@ -347,7 +346,7 @@ impl AppRunner {
                 self.app.clear_color(&self.egui_ctx.global_style().visuals),
                 &clipped_primitives,
                 self.egui_ctx.pixels_per_point(),
-                &textures_delta,
+                &mut self.textures_delta,
                 screenshot_commands,
             ) {
                 log::error!("Failed to paint: {}", super::string_from_js_value(&err));

crates/eframe/src/web/web_painter.rs

@@ -24,7 +24,7 @@ pub(crate) trait WebPainter {
         clear_color: [f32; 4],
         clipped_primitives: &[egui::ClippedPrimitive],
         pixels_per_point: f32,
-        textures_delta: &egui::TexturesDelta,
+        textures_delta: &mut egui::TexturesDelta,
         capture: Vec<UserData>,
     ) -> Result<(), JsValue>;
 

crates/eframe/src/web/web_painter_glow.rs

@@ -61,13 +61,16 @@ impl WebPainter for WebPainterGlow {
         clear_color: [f32; 4],
         clipped_primitives: &[egui::ClippedPrimitive],
         pixels_per_point: f32,
-        textures_delta: &egui::TexturesDelta,
+        textures_delta: &mut egui::TexturesDelta,
         capture: Vec<UserData>,
     ) -> Result<(), JsValue> {
         let canvas_dimension = [self.canvas.width(), self.canvas.height()];
 
-        for (id, image_delta) in &textures_delta.set {
-            self.painter.set_texture(*id, image_delta);
+        #[expect(clippy::iter_over_hash_type)] // Order doesn't matter here
+        for (id, image_deltas) in textures_delta.set.drain() {
+            for image_delta in image_deltas {
+                self.painter.set_texture(id, &image_delta);
+            }
         }
 
         egui_glow::painter::clear(self.painter.gl(), canvas_dimension, clear_color);
@@ -79,7 +82,8 @@ impl WebPainter for WebPainterGlow {
             self.screenshots.push((image, capture));
         }
 
-        for &id in &textures_delta.free {
+        #[expect(clippy::iter_over_hash_type)] // Order doesn't matter here
+        for id in textures_delta.free.drain() {
             self.painter.free_texture(id);
         }
 

crates/eframe/src/web/web_painter_wgpu.rs

@@ -164,7 +164,7 @@ impl WebPainter for WebPainterWgpu {
         clear_color: [f32; 4],
         clipped_primitives: &[egui::ClippedPrimitive],
         pixels_per_point: f32,
-        textures_delta: &egui::TexturesDelta,
+        textures_delta: &mut egui::TexturesDelta,
         capture_data: Vec<UserData>,
     ) -> Result<(), JsValue> {
         let capture = !capture_data.is_empty();
@@ -210,13 +210,16 @@ impl WebPainter for WebPainterWgpu {
 
         let user_cmd_bufs = {
             let mut renderer = render_state.renderer.write();
-            for (id, image_delta) in &textures_delta.set {
-                renderer.update_texture(
-                    &render_state.device,
-                    &render_state.queue,
-                    *id,
-                    image_delta,
-                );
+            #[expect(clippy::iter_over_hash_type)] // Order doesn't matter here
+            for (id, image_deltas) in textures_delta.set.drain() {
+                for image_delta in image_deltas {
+                    renderer.update_texture(
+                        &render_state.device,
+                        &render_state.queue,
+                        id,
+                        &image_delta,
+                    );
+                }
             }
 
             renderer.update_buffers(
@@ -388,8 +391,9 @@ impl WebPainter for WebPainterWgpu {
         // However, once we called `wgpu::Queue::submit`, it is up for wgpu to determine how long the underlying gpu resource has to live.
         {
             let mut renderer = render_state.renderer.write();
-            for id in &textures_delta.free {
-                renderer.free_texture(id);
+            #[expect(clippy::iter_over_hash_type)] // Order doesn't matter here
+            for id in textures_delta.free.drain() {
+                renderer.free_texture(&id);
             }
         }