zink: handle PIPE_CAP_MAX_VARYINGS
piglit's bin/built-in-constants /home/zmike/src/piglit/tests/spec/glsl-1.30/minimum-maximums.txt
test fails currently due to using the default PIPE_CAP_MAX_VARYINGS
value. It seems like this is supposed to be somehow derived from VkPhysicalDeviceLimits
?
Other drivers set it to MAX_VARYINGS
(32), but for zink, it seems like MAX_VARYINGS / 2
(16) is what we want in order to pass the spec tests, but I'm not sure whether this is correct for anyone but me.