Instead of starting recording blit commands immediately we can
record them closer to where we write to the Image directly. This makes
it a bit easier to move towards hiding the command buffers. And easier
to move to using implicitly generated barriers.

Bug: angleproject:2828
Change-Id: Ida7d4081dd41c8ee19c7dc3fb234df1bb065794d
Reviewed-on: https://chromium-review.googlesource.com/1254382
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>