glsl/linker: Allow fragment output overlap for gl_LastFragData.
glsl/linker: Allow fragment output overlap for gl_LastFragData.

gl_LastFragData overlaps gl_FragData by definition.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler