Shaders: Anime4K at full precision on OpenGL ES; sharp video renders the fitted picture

Colours and grid: media_kit renders through OpenGL ES, where mpv's
mediump samplers made Anime4K's networks compute at half precision
(fp16 on NVIDIA); the VL networks of HQ overflow and round there,
strongest once the upscalers run at screen size. The extracted copies
route every network read through a highp identity function; on GLES the
result now equals desktop OpenGL's.

Resize: the output is the picture fitted in the view (no bars, centred
on whole pixels), so Flutter scales the last texture to the new view
without the shrink-then-jump; the resize wait is 120 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -29,6 +29,8 @@ Links you put up are downloaded once for everyone, quick chat and danmaku in the
 
 ### Fixed
 
+- Upscaling shaders (Anime4K, the HQ modes) no longer shift the colours or draw a grid over the picture, which showed most in a large window or fullscreen
+- Sharp video: resizing the window or opening the chat panel no longer shrinks the picture for a moment before it jumps back
 - Joining while something plays on Windows: the video opens again by itself when it was too early
 - Relay: players no longer get stuck after a slow or stalled source
 - Relay: X (Twitter) clips play through the relay