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/builtin_functions.cpp

@@ -493,6 +493,7 @@ private:
 /** Flags to _texture() */
 #define TEX_PROJECT 1
 #define TEX_OFFSET  2
+#define TEX_COMPONENT 4
 
    ir_function_signature *_texture(ir_texture_opcode opcode,
                                    builtin_available_predicate avail,
@@ -3322,6 +3323,18 @@ builtin_builder::_texture(ir_texture_opcode opcode,
       tex->offset = var_ref(offset);
    }
 
+   if (opcode == ir_tg4) {
+      if (flags & TEX_COMPONENT) {
+         ir_variable *component =
+            new(mem_ctx) ir_variable(glsl_type::int_type, "comp", ir_var_const_in);
+         sig->parameters.push_tail(component);
+         tex->lod_info.component = var_ref(component);
+      }
+      else {
+         tex->lod_info.component = imm(0);
+      }
+   }
+
    /* The "bias" parameter comes /after/ the "offset" parameter, which is
     * inconsistent with both textureLodOffset and textureGradOffset.
     */