GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
A simple, GLSL shader-based blur effect.
The blur shader is taken straight from the test-shader.c interactive
test case. It's a fairly clunky, inefficient and visually incorrect
implementation of a box blur, but it's all we have right now until I
figure out a way to do multi-pass shading with the current API.
