glsl/linker: always validate explicit location among inputs

glsl/linker: always validate explicit location among inputs

Outputs are always validated when having explicit locations and we
were trusting its outcome to catch similar problems with the inputs
since, in case of having undefined outputs for existing inputs, we
would be already reporting a linker error.

However, consider this case:

  " Shader stage n:
    ---------------

    ...

    layout(location = 0) out float a;

    ...

    Shader stage n+1:
    -----------------

    ...

    layout(location = 0) in float b;
    layout(location = 0) in float c;

    ...
  "

Currently, this won't report a linker error even though location
aliasing is happening for the inputs.

Therefore, we also need to validate the inputs independently from the
outcome of the outputs validation.

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>