zink: rework input/output location emission

zink: rework input/output location emission

glsl builtins that have no analog in spirv are emitted as regular varyings,
which means they take up a slot.

we need to ensure that there's no conflict between these regular varying
slots (from user-defined varyings) and the glsl translated builtins, so
we do that by "reserving" the max number of varying slots that can be used
by a given stage, then remapping all glsl builtins with no spirv builtin
to a packed layout location that can be consistent across stages

sort of addresses mesa/mesa#3113 except now there's 10 fewer varying slots

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5432>