glsl/linker: check for xfb_offset aliasing

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>

src/compiler/glsl/link_varyings.h

@@ -34,7 +34,7 @@
 
 #include "main/glheader.h"
 #include "program/prog_parameter.h"
-
+#include "util/bitset.h"
 
 struct gl_shader_program;
 struct gl_shader;
@@ -99,7 +99,9 @@ public:
    bool store(struct gl_context *ctx, struct gl_shader_program *prog,
               struct gl_transform_feedback_info *info, unsigned buffer,
               unsigned buffer_index, const unsigned max_outputs,
-              bool *explicit_stride, bool has_xfb_qualifiers) const;
+              BITSET_WORD *used_components[MAX_FEEDBACK_BUFFERS],
+              bool *explicit_stride, bool has_xfb_qualifiers,
+              const void *mem_ctx) const;
    const tfeedback_candidate *find_candidate(gl_shader_program *prog,
                                              hash_table *tfeedback_candidates);