glsl: update parser to allow duplicate default layout qualifiers

glsl: update parser to allow duplicate default layout qualifiers

In order to only create a single node for each default declaration
we add a new boolean parameter to the in/out merge function to
only create one once we reach the rightmost layout qualifier.

From the ARB_shading_language_420pack spec:

   "More than one layout qualifier may appear in a single
   declaration. If the same layout-qualifier-name occurs in
   multiple layout qualifiers for the same declaration, the
   last one overrides the former ones."

Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>