The issue was going to the fact that GL_ALPHA was deprecated in core profile
and common solution online is to use GL_RED instead. That is what is done in
this commit.
Ignore texture matrix in the shader, stop messing with texture matrix in BLF code.
Use linear screen-space interpolation instead of perspective.
Avoid redundant call to glMatrixMode.
With USE_GLSL enabled, GPU_basic_shader(TEXTURE|COLOR) always rendered black. New shader uses a solid color + alpha channel of texture (which in our case is a font glyph). See fragment shader for details.
I prefer this approah -- multiple shaders that each do one thing well (and are easy to read/write/understand), instead of one shader that can do many things given the right options.
