Front-end: Complete GL_ARB_compute_shader implementation.

Core compute shaders were working, but the extension wasn't implemented.

Test/420.comp

@@ -4,4 +4,19 @@ layout(local_size_x = 2) in;  // ERROR, no compute
 
 #extension GL_ARB_compute_shader : enable
 
-layout(local_size_x = 2) in;
+layout(local_size_x = 2, local_size_y = 4, local_size_z = 6) in;
+
+shared vec3 sfoo;
+
+void main()
+{
+    sfoo = vec3(gl_WorkGroupSize.x, gl_WorkGroupSize.y, gl_WorkGroupSize.z);
+    sfoo += gl_WorkGroupSize + gl_NumWorkGroups + gl_WorkGroupID + gl_LocalInvocationID + gl_GlobalInvocationID;
+    sfoo *= gl_LocalInvocationIndex;
+    sfoo += gl_MaxComputeWorkGroupCount + gl_MaxComputeWorkGroupSize;
+    sfoo *= gl_MaxComputeUniformComponents +
+            gl_MaxComputeTextureImageUnits +
+            gl_MaxComputeImageUniforms +
+            gl_MaxComputeAtomicCounters +
+            gl_MaxComputeAtomicCounterBuffers;
+}