Minor OpenGL improvements

• Made skincolor blends more even for colors that have duplicate indices
• Boss flashes are more accurate to how Software does them -- in particular, TC_METALSONIC now remaps the blend image to Cobalt, then inverts the dark blue (but leaves black alone). This means that Metal Sonic boss can use a similar blend image as his character & it will look identical to his character.
• All blends that don't use the blend image at all can now work without needing to supply one (not sure if I implemented this well, please scrutinize)
• Added the fake contrast effect (https://doomwiki.org/wiki/Fake_contrast). Can be turned back off with gr_fakecontrast.
• Transparent planes are fullbright unless if a colormap setting is applied, ala Software

Skincolor blend changes

Before:

After:

Fake contrast

Before:

After:

New Metal Sonic boss look