Match image file extensions case-insensitively (#8430)

`image.PNG` and `cat.SVG` were not recognized as images.

Adds `egui::load::has_extension(uri, extension)`, which ignores ASCII
case and any `#fragment`, and uses it for the `.svg`, `.gif`, `.webp`
and `.png` checks.

Note: gif/webp URIs like `a#b.gif` no longer match, since the fragment
is now excluded.

* [x] I have followed the instructions in the PR template

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

crates/eframe/src/native/glow_integration.rs

@@ -1720,7 +1720,7 @@ fn save_screenshot_and_exit(
     screen_size_in_pixels: [u32; 2],
 ) {
     assert!(
-        path.ends_with(".png"),
+        egui::load::has_extension(path, "png"),
         "Expected EFRAME_SCREENSHOT_TO to end with '.png', got {path:?}"
     );
     let screenshot = painter.read_screen_rgba(screen_size_in_pixels);