Add support for loading less common DDS formats

[BlueCube3310]

Follow-up to #81220

Implements support for loading some less common DDS formats, such as:

• A8,
• DXT2,
• DXT4,
• B2GR3,
• B2GR3A8,
• LA4,
• RGB32F,
• SRGB variants of RGBA8, BC1, BC2, BC3 and BC7

Also changes the loader to now require the DDPF_LUMINANCE flag for grayscale formats, though there is a chance this may break files created by writers which don't set it.

I've marked it as draft in case there are more unsupported formats.

MRP for testing the changes: DDSFormatsUncommon.zip

[fire]

If we say we support DDS, it makes sense that we should support all dds formats.

I don't have production samples to test. There is a test sample. Did not technical review.

[clayjohn]

Looks fine to me. I didn't test locally, but I trust that bluecube3310 has done a good job testing

[akien-mga]

Thanks!