Fix an invalid null deref check in the slider code.
spearmint: Add engine support for using custom dlight shader/image

Based on RTCW. Not exposed to VM yet.
OpenGL2 don't try to dlight surfaces that had all dlights culled

In the renderers, dlightbits are never cleared from world surfaces.
The dlight image does not repeat, so if it draws on extra surfaces it's
not visible.

However if using a repeating image (tr.defaultImage instead of tr.dlightImage);

  * In OpenGL1 image is only drawn on surfaces close to dlight origin.
  * In OpenGL2 image is draw on surfaces clearly outside the dlight radius, including past non-dlighted surfaces.

It seems there was a similar issue with pshadowBits. So update surface
dlightBits even if 0, like already done for pshadowBits. This causes
only surfaces close to origin to be affected. (Though it is a little
farther than in OpenGL1.)

I have no idea why this isn't a problem in OpenGL1.
Merge remote-tracking branch 'ioquake3/master'
spearmint: Allow refdef_t to drop non-sky surfaces

RTCW SP drops skybox portal ground surfaces if RDF_DRAWSKYBOX is not set.
Add a simpler non-skybox specific RDF_ONLYSKY instead.

Also make RDF_NOSKY/RDF_ONLYSKY include "surfaceParm sky" shaders,
instead of just shaders with skyParms or FAKK's skyportal.

Drop the surfaces before checking dlights, sorting the surfaces, etc
instead of right before rendering.
spearmint: Don't mark portalsky shaders as isSky

I made portalsky set isSky so that RDF_NOSKY would drop them.
Since then I made RDF_NOSKY drops surfaceParm sky (which all
FAKK's portalsky shaders have), so it's not needed.

Also the one stage in the portalsky shaders doesn't seem to draw
anything (blendFunc GL_ZERO GL_ONE).
spearmint: Add support for JA's rgbGen lightingDiffuseEntity

rgbGen lightingDiffuseEntity. Lighting diffuse times entity RGB color.

Seems darker in opengl2 than opengl1, but might just be lighting is darker in gernal or something.
Not supported on altivec because I cannot understand the code by looking at it.