Post processor skybox

Post processing no longer done in the skybox.
OpenGL remove HOMs

Removes HOMs from OpenGL.
This also seems to have made skyboxes not draw ontop of everything else
on Intel GPU's.
OpenGL Flashpal Flashes

Fixed flashes for OpenGL Nukes/Teleports etc.
Let steal some OpenGL fixes

Merge branch 'master' of https://github.com/Sryder13/SRB2