Zink fragcolor lowering

Mike Blumenkrantz requested to merge zmike/mesa:zink-fragcolor_lowering into master

this handles gl_FragColor in ntv in a way that translates out to useful code on the vk side when multiple render targets are used, preserving EXT_multiview_draw_buffers semantics

while all gl_FragData[n] members are emitted regardless of the number of render targets in use, these end up getting pruned by nir anyway, which saves us from needing to use shader keys here to check the number of cbufs bound

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