glsl: add basic KHR_blend_equation_advanced infrastructure

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/compiler/glsl/glsl_parser_extras.h

@@ -649,6 +649,8 @@ struct _mesa_glsl_parse_state {
 
    /* KHR extensions go here, sorted alphabetically.
     */
+   bool KHR_blend_equation_advanced_enable;
+   bool KHR_blend_equation_advanced_warn;
 
    /* OES extensions go here, sorted alphabetically.
     */