i965/gs: Fixup gl_PointSize on entry to geometry shaders.

gl_PointSize is stored in the w component of VARYING_SLOT_PSIZ, but
the geometry shader infrastructure assumes that it should look for all
geometry shader inputs of type float in the x component.  So when
compiling a geomtery shader that uses a gl_PointSize input, fix it up
during the shader prolog by moving the w component to the x component.

This is similar to how we emit fixups and workarounds for vertex
shader attributes.

Fixes piglit test spec/glsl-1.50/execution/geometry/core-inputs.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp

@@ -156,6 +156,23 @@ vec4_gs_visitor::emit_prolog()
       }
    }
 
+   /* If the geometry shader uses the gl_PointSize input, we need to fix it up
+    * to account for the fact that the vertex shader stored it in the w
+    * component of VARYING_SLOT_PSIZ.
+    */
+   if (c->gp->program.Base.InputsRead & VARYING_BIT_PSIZ) {
+      this->current_annotation = "swizzle gl_PointSize input";
+      for (int vertex = 0; vertex < c->gp->program.VerticesIn; vertex++) {
+         dst_reg dst(ATTR,
+                     BRW_VARYING_SLOT_COUNT * vertex + VARYING_SLOT_PSIZ);
+         dst.type = BRW_REGISTER_TYPE_F;
+         src_reg src(dst);
+         dst.writemask = WRITEMASK_X;
+         src.swizzle = BRW_SWIZZLE_WWWW;
+         emit(MOV(dst, src));
+      }
+   }
+
    this->current_annotation = NULL;
 }