zink: handle partial writes to shader outputs

zink: handle partial writes to shader outputs

this is super gross. spirv doesn't provide any facility for doing per-component
writes, which means all components of a value must be written every time

to this end, we need to manually split both the src and dst composites and
do per-component access for each store in order to accurately handle both
non-sequential wrmasks (which could be handled by nir_lower_wrmasks, yes, but
we aren't using it) as well as partial wrmasks

see also mesa/mesa#4006

Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8152>