Fix signatures of GPU runtime functions in a few more places
Add error handling to halide_opengl_init
Request RGBA8 pixelbuffer from X11

I'm not sure this is stricly necessary, but it can't hurt to be more explicit
about our requirements.
Remove OpenGL test for uint16 support

16bpp textures don't seem to be widely supported, so remove the test for
now. I think we will need floating point textures before we can support this
in a portable way.