Added ClearDecals export.
[Rend2] Fix lightingDiffuseEntity shader keyword.

The lightingDiffuseEntity keyword was being silently ignored. This commit
restores its functionality to the same as the vanilla renderer.
[Rend2] Fix surface sorting for post-renders.

Surfaces with the renderfx bit RF_DISTORTION, RF_FORCEPOST or
RF_FORCE_ENT_ALPHA should be drawn after all other surfaces. This commit
sorts the surfaces properly such that these surfaces are drawn last.