Uses the GL_ARB_blend_func_extended to get single-pass, per-channel
alpha blending. gl_generator is now used instead of gl to enable the
extension.
The background color is removed since that presumably needs to run in a
separate pass.
OpenGL only supports shared alpha blending. Subpixel font rendering
requires using the font RGB values as alpha masks for the corresponding
RGB channels. To support this, blending is implemented in the fragment
shader.
- Commend vertex slice
- Add helper for binding mask texture (and specify that it's a mask)
- Prefix uniform members of ShaderProgram with u_. This makes it easy to
identify in the rest of code.
