glsl: allow duplicate layout-qualifier-names

glsl: allow duplicate layout-qualifier-names

This is added by ARB_enhanced_layouts although it doesn't fit
into any of the six main changes so we enable this independently.

From the ARB_enhanced_layouts spec:

   "More than one layout qualifier may appear in a single
   declaration. Additionally, the same layout-qualifier-name
   can occur multiple times within a layout qualifier or across
   multiple layout qualifiers in the  same declaration"

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