Add R8_UNORM and R16_UNORM extended surface formats

[kg]

Tested on OpenGL, D3D11 and SDL_GPU.

[kg]

You may ask "Why do we need R8_UNORM when Alpha8 exists?" And the answer is that Alpha8 behaves differently on each backend right now, but R8_UNORM as Red is consistent.

[flibitijibibo]

If the behavior is different that's likely a bug, unless the APIs define them differently - I would definitely prefer bugs first and extensions dead last, rather than the other way around.

[kg]

I looked into fixing Alpha8 before and couldn't figure it out. I'd be happy to pair with someone who wants to look into it I guess? It's also a problem in that Alpha8 RTs don't work in GL, while R8/R16 ones do.

[flibitijibibo]

Blocked by MoonsideGames/dav1dfile#4

[flibitijibibo]

We're close to the end of FNA's Extension model, but we don't want to block extensions in progress, so I'm doing an approval system... if you get a lot of e-mails, apologies for that! Anyway, for this one:

This one is pretty straightforward but formats tend to have a problem where we don't check the swizzling closely enough and it leads to pretty nasty compatibility breakages, so we really need a working and publicly available example to make sure that we're doing this right and that someone who wants to use this can immediately see how the format is supposed to work.