Fix for the fix for buffer overflow.

k addresses every 2nd row in the grid, so instead of k + 1 => k
the right change is k + 1 => k - 1.
Remove support for paintColor attribute.

This wasn't used in any shaders anyway.
Remove origin vector from gridPoint struct.

12 bytes per Point that can be easily recomputed is a bit wasteful
for a hardly used debug option.
Pack gridPoint struct into 2 * 4 bytes.

The 8 values were chosen so that trilinear interpolation gives
physically reasonable results. Also 8 bit precision seems to be
enough for the light grid, which is a coarse approximation to real
lighting in the first place.
Upload light grid to 3D textures.
Add light grid sampling to the vertexLighting_DBS_entity shader.
Remove unused light uniforms from vertexLighting_DBS_entity shader
Add light grid shading to the vertexLighting_DBS_world shader.

This is used for polygons like creep and bullet marks.
Remove unused AmbientLight and DirectedLight attributes.
Fix error in Z_Free().
Remove last remaining uses of the lightdirection attribute.
Merge branch 'master' into vertexpack
Merge branch 'vertexpack' into for-0.25

Conflicts:
	src/engine/renderer/gl_shader.h