Fix rendering with a triangle strip.

The shape was not a quad, because the triangle strip needs the
vertices in order 1-2-4-3 instead of 1-2-3-4.
Implement Sprite#opacity.
Update cmakefile.
Get rid of depth testing, it broke alpha blending.

anything that was behind a sprite with a greater z-index, even if
it was half transparent wasn't drawn, so the sprite just faded to
black.