Add RSF_FORCE_LIGHTMAP

[VReaperV]

Cgame-side pr: Unvanquished/Unvanquished#3306

Replaces RSF_NOLIGHTSCALE.

If this flag is used when registering a shader, all ST_COLORMAP stages will be changed to ST_DIFFUSEMAP. This is intended for use with particles and trails, to make them use the lightGrid in the GLSL code.

Improves performance with particle systems and trails, e. g. with firebombs, and a massive performance improvement on map https://users.unvanquished.net/~sweet/pkg/map-jota_3.0.dpk in the outside area.

There's for the most part no difference in how the particles/trails look, the only differences are due to the GLSL lightGrid being more precise.

[slipher]

FWIW the realLight on Jota doesn't even do anything currently. The shader must have an rgbgen vertex or entity for the entity color to have any effect. There are only 4 maps where realLight is actually used with a compatible shader:

• gantry (blood drops from fan)
• sectorb17 (smokestack)
• nova (purports to be smoke; haven't tested)
• map-edge1_v3_ye_m.pk3 (purports to be smoke; haven't tested)

Haven't read the code yet but the approach sounds fine.