Instead of unconditonally ending the RenderPass we keep a set of
active XFB buffers in the ContextVk. This lets us re-use RPs when
we don't write to the same buffer repeatedly.

Reduces the RenderPass count in our Manhattan capture from 29->23.

Bug: angleproject:4622
Change-Id: I28c2d4d3db1490e5d07be3c48d21fd2cc6ff85d6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2196957Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>