Frames: cut each frame into its own GPU texture, no more fill matrix (#47)

A frame drawn straight from the sheet showed a hairline of the neighbouring
cell along any edge that fell inside a screen pixel: with antialias on, a
partly covered pixel is still evaluated at its centre, past the edge. On top
of that Pixi rewrites a Graphics fill texture from clamp-to-edge to repeat at
draw time (generateTextureFillMatrix.mjs), so the top row of a sheet read its
bottom row.

TextureLibrary.get(ref) now cuts the rectangle out of the loaded sheet with an
OffscreenCanvas on first use and uploads the cut (key file|x,y,w,h, kept for
the session). Address mode is mirror-repeat: Pixi only rewrites clamp-to-edge,
and within a texel of the border mirror behaves like clamp, for linear as well
as nearest. Loaded sheets get it too, since whole pictures wrapped at their own
borders the same way. viewport/textureFill.ts and its test are gone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -37,7 +37,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 723 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 725 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs