glsl: avoid linker and user varying location to overlap
glsl: avoid linker and user varying location to overlap

Current behavior on the interface matching:

layout (location = 0) out0; // Assigned to VARYING_SLOT_VAR0 by user
out1; // Assigned to VARYING_SLOT_VAR0 by the linker

New behavior on the interface matching:

layout (location = 0) out0; // Assigned to VARYING_SLOT_VAR0 by user
out1; // Assigned to VARYING_SLOT_VAR1 by the linker

v4:
* Fix variable name in assert

Signed-off-by: Gregory Hainaut <874ed946d2e42378b22d113f3a5c5d90141d0a46@gmail.com>
Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler