glsl: Add support for specifying the component in textureGather

ARB_gpu_shader5 introduces new variants of textureGather* which have an
explicit component selector, rather than relying purely on the sampler's
swizzle state.

This patch adds the GLSL plumbing for the extra parameter.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/opt_tree_grafting.cpp

@@ -275,7 +275,6 @@ ir_tree_grafting_visitor::visit_enter(ir_texture *ir)
    switch (ir->op) {
    case ir_tex:
    case ir_lod:
-   case ir_tg4:
    case ir_query_levels:
       break;
    case ir_txb:
@@ -297,6 +296,10 @@ ir_tree_grafting_visitor::visit_enter(ir_texture *ir)
 	  do_graft(&ir->lod_info.grad.dPdy))
 	 return visit_stop;
       break;
+   case ir_tg4:
+      if (do_graft(&ir->lod_info.component))
+         return visit_stop;
+      break;
    }
 
    return visit_continue;