Expose Bayer 2/4 dither modes and add Bayer 16 dither mode

[mvaligursky]

Fixes #6097. The engine already had bayer2/bayer4 GLSL/WGSL helper functions implemented but unreachable, and no way to get a coarser or finer ordered-dither pattern than the existing Bayer 8. This adds DITHER_BAYER2 and DITHER_BAYER4 as selectable opacity dither modes, and introduces a new DITHER_BAYER16 mode for finer dithering.

Changes:

• Add DITHER_BAYER2 and DITHER_BAYER4 constants, wiring up the existing bayer2()/bayer4() shader functions to the opacityDither chunk (GLSL + WGSL)
• Add DITHER_BAYER16 constant and a new bayer16() function (GLSL + WGSL), extending the existing Bayer matrix recursion
• Update StandardMaterial#opacityDither / #opacityShadowDither JSDoc to document the new modes

API Changes:

• New exports: DITHER_BAYER2, DITHER_BAYER4, DITHER_BAYER16

Examples:

• materials/material-transparency: added Bayer2, Bayer4 and Bayer16 columns alongside the existing dither modes
• graphics/dithered-transparency: added Bayer2, Bayer4 and Bayer16 to the Dither Color / Dither Shadow dropdowns

[github-actions]

Public API report

This PR changes the public API surface (+3 / −0), per the docs' rules (@ignore / @Private / undocumented are excluded).

Show API diff

+const DITHER_BAYER16: "bayer16"
+const DITHER_BAYER2: "bayer2"
+const DITHER_BAYER4: "bayer4"

Informational only — this never fails the build.

[github-actions]

Build size report

This PR changes the size of the minified bundles.

Bundle	Minified	Gzip	Brotli
playcanvas.min.js	2325.6 KB (+1.7 KB, +0.07%)	598.3 KB (+0.3 KB, +0.05%)	465.2 KB (+0.2 KB, +0.05%)
playcanvas.min.mjs	2323.0 KB (+1.7 KB, +0.07%)	597.1 KB (+0.3 KB, +0.05%)	464.7 KB (+0.4 KB, +0.09%)