gallium: add TGSI_SEMANTIC_VERTEXID_NOBASE and TGSI_SEMANTIC_BASEVERTEX

Plus a new PIPE_CAP_VERTEXID_NOBASE query. The idea is that drivers not
supporting vertex ids with base vertex offset applied (so, only support
d3d10-style vertex ids) will get such a d3d10-style vertex id instead -
with the caveat they'll also need to handle the basevertex system value
too (this follows what core mesa already does).
Additionally, this is also useful for other state trackers (for instance
llvmpipe / draw right now implement the d3d10 behavior on purpose, but
with different semantics it can just do both).
Doesn't do anything yet.
And fix up the docs wrt similar values.

v2: incorporate feedback from Brian and others, better names, better docs.

Reviewed-by: Brian Paul <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>
Reviewed-by: Marek Olšák <[email protected]>

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