mesa/cs: Handle compute shaders in _mesa_use_program().

v2: do cs after the ordered pipeline stages for consistency.
mesa/cs: Create the gl_compute_program struct, and the code to initialize it.
mesa/cs: Implement MAX_COMPUTE_WORK_GROUP_SIZE constant.
mesa/cs: Implement MAX_COMPUTE_WORK_GROUP_INVOCATIONS constant.
mesa/cs: Implement MAX_COMPUTE_WORK_GROUP_COUNT constant.
glsl/cs: Handle compute shader local_size_{x,y,z} declaration.
mesa/cs: Handle compute shader local size during linking.
main/cs: Implement query for COMPUTE_WORK_GROUP_SIZE.
glsl/cs: Prohibit user-defined ins/outs in compute shaders.
glsl/cs: Prohibit mixing of compute and non-compute shaders.

Fixes piglit test:
spec/ARB_compute_shader/linker/mix_compute_and_non_compute
i965/cs: Create the brw_compute_program struct, and the code to initialize it.
i965/cs: Allow ARB_compute_shader to be enabled via env var.

This will allow testing of compute shader functionality before it is
completed.

To enable ARB_compute_shader functionality in the i965 driver, set
INTEL_COMPUTE_SHADER=1.
======= More ARB_compute_shader work =======
main: Clean up a strange construction in use_shader_program().
mesa/cs: Add DispatchCompute() to driver function table.
main/cs: Implement front end code for glDispatchCompute().
WIP: implement DispatchCompute() back-end