From page 76 (page 80 of the PDF) of the GLSL 4.60 v.5 spec:

  " No aliasing in output buffers is allowed: It is a compile-time or
    link-time error to specify variables with overlapping transform
    feedback offsets."

Currently, this is expected to fail, but it succeeds:

  "

    ...

    layout (xfb_offset = 0) out vec2 a;
    layout (xfb_offset = 0) out vec4 b;

    ...

  "

Fixes the following piglit test:
tests/spec/arb_enhanced_layouts/compiler/transform-feedback-layout-qualifiers/xfb_offset/invalid-overlap.vert

Fixes the following test:
KHR-GL44.enhanced_layouts.xfb_output_overlapping

v2:
  - Use a data structure to track the used components instead of a
    nested loop (Ilia).

v3:
  - Take the BITSET_WORD array out from the
    gl_transform_feedback_buffer struct and make it local to the
    validation process (Timothy).
  - Do not use a nested scope for the validation (Timothy).

v4:
  - Add reference to the fixed piglit test in the commit log.
  - Add reference to the fixed VK-GL-CTS test in the commit
    log (Tapani).
  - Empty initialize the BITSET_WORD pointers array (Tapani).

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>