llvmpipe: fix layer/vp input into fs when not written by prior stages

ARB_fragment_layer_viewport requires that if a fs reads layer or viewport
index but it wasn't output by gs (or vs with other extensions), then it reads
0. This never worked for llvmpipe, and is surprisingly non-trivial to fix.
The problem is the mechanism to handle non-existing outputs in draw is rather
crude, it will simply redirect them to whatever is at output 0, thus later
stages will just get garbage. So, rather than trying to fix this up (which
looks non-trivial), fix this up in llvmpipe setup by detecting this case there
and output a fixed zero directly.
While here, also optimize the hw vertex layout a bit - previously if the gs
outputted layer (or vp) and the fs read those inputs, we'd add them twice
to the vertex layout, which is unnecessary.
And do some minor cleanup, slots don't require that many bits, there was some
bogus (but harmless) float/int mixup for psize slot too, make the slots all
unsigned (we always put pos at pos zero thus everything else has to be positive
if it exists), and make sure they are properly initialized (layer and vp index
slot were not which looked fishy as they might not have got set back to zero
when changing from a gs which outputs them to one which does not).

This fixes the failures in piglit's arb_fragment_layer_viewport group
(3 each for layer and vp).

Reviewed-by: Jose Fonseca <[email protected]>

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