glsl: validate output blocks against input blocks

Until now were validating in/out blocks by listing the inputs in the
consumer stage and then, for each output of the producer, we checked that
it was a match if it was consumed. This method does not catch the case
where the consumer has an input that is not present as an output in the
producer stage, because it only generates link errors for outputs present
in the producer stage that don't match the inputs in the consumer stage.
The current method does catch the case were an output from the producer
stage is not consumed, which is irrelevant and is ignored.

By reversing the way we do this, we can detect this situation, so this
patch lists the outputs of the producer stage and then validates inputs
of the consumer stage against them. If we see an input in the consumer
for which there is no associated output in the producer, we produce a
link error.

The only exception to this is the special built-in input block gl_in[],
since this is implicitly generated for geometry and tessellation stages,
but we don't generate it if the producer stage does not write to any of
the pre-defined outputs (for example, if the vertex shader does not write
to gl_Position, etc). Since writing to these is not mandatory, do not
produce a link error in that case. There is a CTS tessellation test
(GL45-CTS.tessellation_shader.program_object_properties) that has an
empty vertex shader (so it does not produce gl_in[]) and would fail to
link if we don't do this.

This fixes the following dEQP test:
dEQP-GLES31.functional.shaders.linkage.io_block.missing_output_block

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98245
Reviewed-by: Nicolai Hähnle <[email protected]>

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