Round interpolated texture IDs before texture and resolution lookup

[AyyoubAha]

This PR

• Fixes a bug

Round the interpolated texture ID before selecting a texture or its resolution. An ID slightly above or below an integer previously fell through the exact-equality checks, returning a transparent sample or a zero resolution. Both helpers now use floor(outTexDatum + 0.5). Single-texture batches and the fallback for out-of-range integer IDs are unchanged.

Based on DainDwarf's report and suggested rounding approach in #7372. Related to #7372; the original SwiftShader rotation reproduction still needs confirmation with this patch.

Validation:

• Added tests/renderer/webgl/texture-id-regression.html, a manual WebGL 1 test that compiles the actual generated GLSL helpers and reads back pixels. It supplies exact IDs, positive/negative perturbations, and out-of-range integer controls; tests texture sampling and resolution lookup, mediump/highp, and 1, 2, and the device maximum number of texture units. Serve the repository root over HTTP, open the file, and click Run shader checks.
• On Intel Graphics / ANGLE D3D11, Chromium 152, 16 texture units: before the change, 144 checks failed and 108 passed; after the change, all 252 passed. The non-integer inputs are deliberate fault injection, not a claim that this GPU reproduces the reported interpolation drift.
• The original 120-text example renders intact at 1° and 45° on this Intel GPU, with both the released 4.2.1 build and the patched build. No SwiftShader, mobile GPU, or performance benchmark was run.
• npm test -- tests/renderer/webgl --maxWorkers=2: 69 files / 1,544 tests passed. An existing circular-dependency warning was emitted.
• npm run build: webpack compiled successfully. The optional copy step reports that the sibling examples repository is absent.
• Targeted ESLint 8.57.1 with the repository's existing configuration and --no-ignore passed for both changed generated JavaScript files. The repository-wide npm run lint fails to load .eslintrc.json under its installed ESLint 10 (ERR_IMPORT_ATTRIBUTE_MISSING); this PR does not alter dependencies or lint configuration.
• git diff --check passed.

The GLSL sources and their generated JavaScript modules are included; README, changelog, and distribution bundles are unchanged.