perf(textures): let textureCompression tree-shake when the flag is off

[chiefcll]

What

ENABLE_COMPRESSED_TEXTURES already gated the compressed load path, but two other call sites referenced textureCompression.js unconditionally, so a bundler could never drop the module — even with __enableCompressedTextures__ defined as false.

Both are now behind the flag:

• ImageTexture.getTextureSource() — the Canvas2D fail-fast check called isCompressedTextureContainer() outside any guard.
• WebGlCtxTexture.onLoadRequest() — the 'mipmaps' in tdata upload branch called uploadCompressedTexture[type].

Why

With the flag off, no TextureData can carry mipmaps (only loadCompressedTexture produces them) and no compressed source can load, so both branches are provably dead. package.json already declares sideEffects: false, so once esbuild folds the constant to false, Rollup drops the branches and then the whole module.

Verification

Real Vite lib build importing both files from dist/:

__enableCompressedTextures__	bundle	references to compression symbols
false	28.57 kB	0
true	43.16 kB	9

~14.6 kB raw (~3.3 kB gzip) removed from apps that don't opt in.

pnpm build clean, 360 unit tests pass, prettier/eslint clean on both files.

Reviewer notes

• Elimination depends on the consumer's bundler constant-folding the flag. A consumer that never defines __enableCompressedTextures__ leaves typeof __enableCompressedTextures__ !== 'undefined' as a runtime check and retains the module — same behavior as USE_RTT and the other flags in src/utils.ts. Compressed textures still correctly no-op at runtime, just without the size win.
• With the flag off, a type: 'compressed' source in Canvas2D no longer gets the explicit "not supported in Canvas2D render mode" error — it falls through to loadImage and fails there instead. That path was already unreachable-by-design when the flag is off, so this changes the error message, not the outcome.

🤖 Generated with Claude Code