glsl: ignore all but the rightmost layout-qualifier-name

When a layout contains a duplicated layout-qualifier-name in a single
declaration, only the last occurrence should be taken into account.

From page 59 (page 65 of the PDF) of the GLSL 4.40 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. When the same
    layout-qualifier-name occurs multiple times, in a single
    declaration, the last occurrence overrides the former
    occurrence(s)."

Consider this example:

  " #version 150
    #extension GL_ARB_enhanced_layouts: enable

    layout(max_vertices=2, max_vertices=3) out;
    layout(max_vertices=3) out;"

Although different values for "max_vertices" results in a compilation
error. The above code is valid because max_vertices=2 is ignored.

When merging qualifiers in an ast_type_qualifier, we now simply ignore
new appearances of a same layout-qualifier-name if the
"is_single_layout_merge" parameter is true, this works because the GLSL
parser processes qualifiers from right to left.

Reviewed-by: Timothy Arceri <[email protected]>
Signed-off-by: Andres Gomez <[email protected]>

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