Betsy: Implement BC4 compression

[BlueCube3310]

Adds support for BC4 / ATI1 / RGTC_R compression to Betsy. This is a prerequisite for BC3/BC5, since they use a separate BC4 block for storing their uncorrelated alpha and green channels respectively.
In terms of performance, it is comparable to etcpak.

TODO:

• Add a testing project,
• Document the changes.

Future work:

• Add BC3 and BC5,
• Convert RGB-RGBA on the GPU,
• Optimize mipmap compression,

MRP: betsyrgtcr.zip

[fire]

@Calinou How do I queue this to run on the benchmark servers? Also can you take a look?

[clayjohn]

Looks great!

[Calinou]

How do you get a texture to import as BC4 in Godot? I've tried to import a grayscale PNG and a RGB PNG that only has a red channel (green/blue are fully black as confirmed in GIMP). However, both are imported as DXT1 with VRAM compression or BPTC RGBA with high-quality enabled (neither have an alpha channel).

test_vram_red_grayscale.zip

[BlueCube3310]

How do you get a texture to import as BC4 in Godot? I've tried to import a grayscale PNG and a RGB PNG that only has a red channel (green/blue are fully black as confirmed in GIMP). However, both are imported as DXT1 with VRAM compression or BPTC RGBA with high-quality enabled (neither have an alpha channel).

test_vram_red_grayscale.zip

BC4 only works with red only images with the channel packing set to optimized

[Repiteo]

Thanks!