web: Avoid panic from lost texture updates when loaded on a background tab (#8313)

The `textures_delta` in `AppRunner::logic()` comes from
`TextureManager::take_delta()` via `end_pass()`. Previously, if
`is_visible` was `false`, these deltas would have been dropped and never
applied, so the `TextureManager` state gets out of sync with the
`Renderer`.

That causes https://github.com/emilk/egui/issues/8228: When the app is
loaded in a background tab and the first frame occurs via the
`setTimeout` path prior to the tab becoming visible, it loses the
`ImageDelta` representing the initial creation of the font atlas
texture. When the user then switches to the tab, it panics at
crates/egui-wgpu/src/renderer.rs:669:18 with `Tried to update a texture
that has not been allocated yet` when attempting to update the texture
that the wgpu renderer never saw when it was created.

Testing: To make it load in a background tab, put `data:text/html,<a
href=http://127.0.0.1:8765/>Click</a>` in the address bar and then
middle click the resulting link. On Chrome you also have to hover the
tab preview before switching to the tab to reproduce the issue.

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* Closes <https://github.com/emilk/egui/issues/8228> and its duplicate
https://github.com/emilk/egui/issues/8278
* [x] I have followed the instructions in the PR template

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/src/web/app_runner.rs

@@ -316,7 +316,7 @@ impl AppRunner {
         }
 
         self.handle_platform_output(platform_output);
-        if is_visible {
+        if is_visible || !textures_delta.is_empty() {
             self.textures_delta.append(textures_delta);
             self.clipped_primitives = Some(self.egui_ctx.tessellate(shapes, pixels_per_point));
         }