[Mesa-dev] Allow duplicate layout qualifiers V2

[Timothy Arceri]

After sending out V1 where I didn't fix the global default
merging (just issued and error), I got motivated and fixed
it properly. 

There are two changes here.

1. ARB_enhanced_layouts

Allows duplicates within a single layout qualifier e.g.

layout(location = 0, location = 1) out vec4 a;

2. ARB_shading_language_420pack

Allows multiple layout qualifiers e.g.

layout(location = 0) layout(location = 2) out vec4 b;


I've Cc'ed Matt as he gave feedback on my original change but
I would be greatful to anyone who takes a look at these.

Thanks, Tim.