- Don't assume GLSL 4.30 is available just because the driver
supports compute shaders. Drivers before OpenGL 4.3 may still
offer the extension.
- Use GLSL 1.30 by default, and turn on additional features
using extensions. Skip any tests requiring extensions that
aren't supported by the driver.
- Unsigned literal ints should have a 'u' suffix.
- Clean up a few dead Python expressions