glsl: ignore all but the rightmost layout qualifier name from the rightmost layout...

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

From page 46 (page 52 of the PDF) of the GLSL 4.20 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."

Consider this example:

  " #version 150
    #extension GL_ARB_shading_language_420pack: enable

    layout(max_vertices=2) layout(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.

Hence, when merging qualifiers in an ast_type_qualifier, we now ignore
new appearances of a same layout-qualifier-name if the new
"is_multiple_layouts_merge" parameter is on, since the GLSL parser
works in this case from right to left.

In addition, any special treatment for the buffer, uniform, in or out
layout defaults has been moved in the GLSL parser to the rule
triggered just after any previous processing/merging on the
layout-qualifiers has happened in a single declaration since it was
run too soon previously.

Fixes GL44-CTS.shading_language_420pack.qualifier_override_layout

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>