VaVDA: introduce enum BufferAllocationMode

This CL removes |decode_using_client_picture_buffers_| and
|use_reduced_number_of_allocations_| in favour of a new enum class
BufferAllocationMode and associated member |buffer_allocation_mode_|,
filled in a single place with verbose explanations as to why and the
associated TODOs.

No new functionality intended: this is purely a refactoring,
hopefully leaving things clearer (but also prevents wrong uses
e.g. |decode_using_client_picture_buffers_| and
|use_reduced_number_of_allocations_| both true, which doesn't
make sense but is hypothetically possible on ToT).

Bug: 912295
Change-Id: I9b3be1cbc1314849e3693ce04196bece3b7f0ed2
Reviewed-on: https://chromium-review.googlesource.com/c/1443979
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628376}
3 files changed