glsl: At link-time, check that globals have matching centroid qualifiers

Fixes bug 31923: http://bugs.freedesktop.org/show_bug.cgi?id=31923
(cherry picked from commit 61428dd2ab)

src/glsl/linker.cpp

@@ -409,6 +409,12 @@ cross_validate_globals(struct gl_shader_program *prog,
 	                           mode_string(var), var->name);
 	       return false;
 	    }
+            if (existing->centroid != var->centroid) {
+               linker_error_printf(prog, "declarations for %s `%s' have "
+                                   "mismatching centroid qualifiers\n",
+                                   mode_string(var), var->name);
+               return false;
+            }
 	 } else
 	    variables.add_variable(var->name, var);
       }