#18 Lower color depth options (8bit, 16bit)

[Vitoria Litwinczyk]

I'm trying to get my Quakespasm to look as crusty as possible, because that's how I like playing older games like this. I think one crucial thing that is missing to finish the look is to be able to lower the color depth below the 24/32 bit colors that Quakespasm.exe and Quakespasm-sdl12.exe allow for.

Adding this feature would nearly complete the retro look for this source port, it would only be missing some texture warping and vertex warble.

Thanks for your time and all the work you all have already put in this sourceport.