Basic support for regional color grading.

A color grading region can be defined by a func_static entity with the
gradingTexture property. The intensity of the grading depends on the
distance between the camera and any brush of the entity, inside the entity
it is always 1, and it decreases linearly to 0 at distance gradingDistance.

The grading entity can be built entirely from hint brushes, they have no
effect on the compiled map, and its brushes shouldn't be too complex as
the distance calculation has to be repeated every frame.