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/ir_print_visitor.cpp

@@ -287,7 +287,6 @@ void ir_print_visitor::visit(ir_texture *ir)
    {
    case ir_tex:
    case ir_lod:
-   case ir_tg4:
    case ir_query_levels:
       break;
    case ir_txb:
@@ -308,6 +307,9 @@ void ir_print_visitor::visit(ir_texture *ir)
       ir->lod_info.grad.dPdy->accept(this);
       printf(")");
       break;
+   case ir_tg4:
+      ir->lod_info.component->accept(this);
+      break;
    };
    printf(")");
 }