Opacity dithering support

[mvaligursky]

Fixes #5159

• Order independent transparency using dithering.
• uses procedural Bayer pattern with 64 dither levels (8x8 grid)
• dithering can be independently enabled for the shadows as well - this works the best with soft shadows such as VSM, otherwise a pattern is more obvious.

new API / example on how to enable:

StandardMaterial.opacityDither = true;
StandardMaterial.opacityShadowDither = true;

material.blendType would ideally be set to pc.BLEND_NONE

dither2.mov

Limitations:

• if two objects are behind each other, with the same opacity, their pattern is the same and that causes the object behind to not be seen. Possible improvement here would be to have some per drew call random ID (meshInstance.id would work well) and the pattern would be offset along x and y based on this number. This is not done in this PR.
• Bayer pattern is used. Other great option without obvious pattern include a blue noise. This was considered, but would require a small blue noise texture to be available, and so this can be done in a separate PR if needed / alternatively the dither chunk can be overriden and implement this.