Change Lightgrid interpolation to match q3 more.

The directed light intensity is interpolated now without consideration of its
direction. That may be physically less plausible, but otherwise directed
lighting can get turned down too much.
Fix lightvector scaling.
Fill in lightgrid samples in walls with interpolated values.

The 3d texture cannot skip samples in walls, so to avoid darkening
of entities near walls and floors there should be light samples inside
walls.
Fix a typo inside ReadLightGrid. This fixes the flickering effect experienced on nvidia hardware.
Normalize r_ambientScale and r_normalScale, remove the CVAR_CHEAT flag of the latter.
Merge branch 'for-0.26' into librocket