Minimal iOS app: Perform present subsequent to flush

Currently the backbuffer present is not on the same MTLCommandQueue
as the flush from Skia, which probably means that it's happening
concurrently. This can overwhelm the GPU. This CL submits the present
to the same queue as the flush.

Change-Id: Ibaf805553931c9dc46368b362a2391425ae3e60e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248557
Reviewed-by: Hal Canary <halcanary@skia.org>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed