rendercheck is a program to test a Render extension implementation against
separate calculations of expected output.

Tests currently include:
- Destination coordinates correctness
- Source coordinates correctness
- Transformed (FilterNearest) source coordinates correctness.
- Composite with and without mask (with/without component alpha), with 1x1
  repeating Pictures and 10x10 Pictures.
- Linear gradients
- Repeating sources/masks at POT and non-POT sizes
- Some regression tests for bugs from freedesktop.org bugzilla.