Metal: Cache render pipelines at the context level Cache Metal render pipelines in a new mtl::PipelineCache which lives at the context level. This allows us to clean up unused pipelines from programs that are not actively used. The cache limits were chosen based on running Chromium. Without a limit, the total number of pipelines peaks at ~200. With frequent GCs, the active working set usually sits at ~60 pipelines. Bug: chromium:1329376 Change-Id: Ifa83b797c893684294e16dd638f6b3a35e1d043f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4608486 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>