WebGPURenderer: correct texelFetch() texel coordinates in WebGL fallback

[aardgoose]

GLSL texelFetch requires integer texel coordinates corrected for coordinate direction using the texture dimensions.

setNode() incorrectly casts the result of uv.setY( ... ) to 'float', attempt to fix that.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	685.83 169.75	685.83 169.75	+0 B +0 B
WebGPU	832.14 222.98	832.24 223.02	+97 B +39 B
WebGPU Nodes	831.65 222.86	831.75 222.9	+97 B +37 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	462.42 111.53	462.42 111.53	+0 B +0 B
WebGPU	529.9 142.75	530 142.79	+97 B +36 B
WebGPU Nodes	486.56 132.61	486.65 132.64	+97 B +31 B