We don't need to use the CommandGraphNode class directly. This CL
consolidates our code so we never call the GraphNodes class directly.
Instead we call operations on GraphResource. This should simplify the
interaction with APIs from the various graph and dependency management
classes in the Vulkan back-end.

A new concept of 'starting' vs 'appending' commands is introduced.
Appending tries to avoid starting new command buffers when possible.

Should not change how the graphs are constructed, and mostly be a
refactoring change. There may be minor behaviour changes to some
commands.

Bug: angleproject:2539
Change-Id: Ia971e5cacb1164b9b3b22fa4a0a55b954d81f10e
Reviewed-on: https://chromium-review.googlesource.com/1052068
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>