Test that compute shader work group sizes are properly bounds checked.
WIP: Test ARB_compute_shader built-in constant gl_WorkGroupSize.
Test ARB_compute_shader built-in constant gl_MaxComputeWorkGroupCount.
Test ARB_compute_shader built-in constant gl_MaxComputeWorkGroupSize.
Test that compute shaders may not have user-defined ins/outs.
shader_runner: Add support for compute shaders.

Currently there is no way for shader_runner to accept output from
compute shaders; however this should still be useful for linker tests.
WIP: compute shader linker tests
WIP: ARB_compute_shader API errors test.