glsl: stop considering unnamed and named structures equal

glsl: stop considering unnamed and named structures equal

Previously, if an unnamed and a named struct contained the same fields,
they were considered the same type during linking of globals.

The discussion around commit e018ea81bf58 ("glsl: Structures must have
same name to be considered same type.") doesn't seem to have considered
this thoroughly, and I see no evidence that an unnamed struct should
ever be considered to be the same type as a named struct.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>