mesa: Add shader storage buffer support to struct gl_context

mesa: Add shader storage buffer support to struct gl_context

This includes the array of bindings, the current buffer bound to the
GL_SHADER_STORAGE_BUFFER target and a set of general limits and default
values for shader storage buffers.

v2:
- Use spec values for the new defined constants (Jordan)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>