Embed the creation of opengl context in the runtime.
Load libGL.so.1 instead of libGL.so
Allow users to set HL_JIT_TARGET, but use host-opengl as fallback
Merge with master and opengl
Sanitize C names to avoid reserved words in C-like languages
Add missing WITH_ARM guards
Potential fix for roundoff errors in pixel normalization logic
Fix rounding behavior of uint8s in shaders
Track include dependencies for runtime .cpp files
Enable support for uint16 textures
Add basic support for error handling in GPU backends

CUDA and OpenCL in particular need more work since they use
halide_assert instead of halide_error+return.
Move OpenGLSupport.cpp to apps/glsl for now
reorder color channels to innermost by default

This reduces the number of interleave/deinterleaves
Corrected type signature in static test
Fix bugs in changes to C name sanitizing.
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.
Create OpenGL context without a Visual