Fix encoding of comments.

This makes some files use utf-8 and fixes a weird quote in another comment.
Apply whitespace fix to SDL2 branch.
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.