mesa: Add GL/GLSL plumbing for ARB_fragment_shader_interlock.

mesa: Add GL/GLSL plumbing for ARB_fragment_shader_interlock.

This extension provides new GLSL built-in functions
beginInvocationInterlockARB() and endInvocationInterlockARB()
that delimit a critical section of fragment shader code. For
pairs of shader invocations with "overlapping" coverage in a
given pixel, the OpenGL implementation will guarantee that the
critical section of the fragment shader will be executed for
only one fragment at a time.

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>