Subpass color clear pipelines are those used to emit partial attachment
clears as draw calls inside the render pass currently bound by the
application in the command buffer, leading to a huge performance improvement
compared to the case where we emit them in their own render pass.
Unfortunately, because the pipeline references the render pass
object in which it is used and the render pass object is owned by the
application (and can be destroyed at any point), we can't cache these
pipelines (unless we implement a refcounting mechanism or other
similar strategy).
Performance impact looks negligible based on experiments with vkQuake3,
probably because the underlying pipeline cache is preventing the
redundant shader recompiles.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>
Iago Toral Quiroga
db0bb36ace