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/egui_kittest/tests/accesskit.rs

@@ -144,7 +144,8 @@ fn accesskit_output_single_egui_frame(run_ui: impl FnMut(&mut Ui)) -> TreeUpdate
     ctx.global_style_mut(|style| style.animation_time = 0.0);
     ctx.enable_accesskit();
 
-    let output = ctx.run_ui(RawInput::default(), run_ui);
+    let mut output = ctx.run_ui(RawInput::default(), run_ui);
+    output.textures_delta.clear(); // Don't panic on drop with unapplied deltas
 
     output
         .platform_output