Switch Metal dynamic buffer allocations to use a ring buffer.

Buffer creation is a big hit on Metal. This allows us to recycle more
of the buffer.

Bug: skia:8243
Change-Id: If2e3b13d8175dfe370bd61771af0a59cce277340
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221120
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed