Palette tonemap mode

[dpJudas]

Pull request: https://github.com/coelckers/gzdoom/pull/86

Adds a Palette tonemap mode that allows gzdoom to get as ugly as zdoom. Generates a 18bpp LUT from the palette of the active game and stores it in a 512x512 texture for the tonemap shader.

Related zdoom forum thread: http://forum.zdoom.org/viewtopic.php?f=19&t=53252

[Rachael]

While we're on the subject - wouldn't it make more sense for the screen flash to be rendered outside of the pre-shader render texture?

It looks rather horrible having the screen flash on the "palette" because a) Original Doom did not do that, and b) Why would you want it calculating inside the other tonemaps, anyway? Also c) They could mess up bloom effects, too.

Nice job on the pull request, though.

[dpJudas]

Yes, it would be more ideal if the flash was done at the post processing stage. Moving the flash to the Present shader would make it do it the same way as zdoom does. The bloom pass actually disables itself when invulnerability is active because otherwise everything would just become a blooming mess.