Metal: Release prov. vertex buffers on event set Fixes a memory leak where provoking vertex buffers would never be released for sequence: 1. draw with flat shading 2. Create EGL fence sync object with Metal event 3. .. 4. Go to 1 Bug: angleproject:8586 Change-Id: I5b302cef82414e4a2886c4a7f570c1bf73a83238 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5349340 Reviewed-by: Kenneth Russell <kbr@chromium.org> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Kenneth Russell <kbr@chromium.org>