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.
Don't change function parameter types for Q_strncpyz in debug builds
Fix undefined totalWeight value crash.

Happens after the map is loaded.
cgame: change cgs.mapname to 'maps/_.bsp' to '_'
renderers: add ScissorSet and ScissorEnable

Used to control temporarily glScissor, the scissor is also modified
by other parts of the renderer.
Adapted from a commit from `Ishq (ded55d8f12e03aceff) + fixes
cgame: Add scissoring support
Reverting the construction of a buildable removes confidence.
Merge remote-tracking branch 'origin/master' into gameplay/viech