Replace our use of r_fakelight and r_equalize_entities_fullbright by r_fullbright_directed.

Unlit maps and g_fullbrightplayers/g_fullbrightitems now will get somewhat different lighting, but should still be easy on the eyes (the former) and stand out properly (the latter).

This will allow simplifying engine code significantly while even being a bit easier on the eyes.

Comparison video: https://www.youtube.com/watch?v=xSuVieA8ILI