Features:
- add GLFW functions to read mouse position/click state
- extend and test comparators in GLSL backend

Broken things:
- Check that types match for varying linking, more thoroughly test it
- Inhibit access to global variables from gpu functions
- Add support for inout parameters to entrypoints, or disallow them

Wishlist:
- multi-dimensional arrays
- images, textures, framebuffers
- boolean matrices in GLSL

