Make clipping apply even with empty clip rects.

Due to weird behavior in SDL2, an empty clip rect is the same as disabling
clipping. To work around this, we use a bogus clip rect (that no pixel in our
viewport will match) when we're passed an empty one.

Fixes GH-116.
Simplify IntersectTHClipRect.
Merge pull request #242 from jorgenpt/sdl2-fix-clipping

Fix clipping issues