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Instead of representing the buffer-queue as a vector of buffer
indices, represent them as a vector of BufferItems (copies).
This allows modifying the buffer slots independent of the queued
buffers.

As part of this change, BufferSlot properties that are only
been relevant in the buffer-queue have been removed.

Also, invalid scalingMode in queueBuffer now returns an error.

ConsumerBase has also changed to allow reuse of the same
buffer slots by different buffers.

Change-Id: If2a698fa142b67c69ad41b8eaca6e127eb3ef75b
Signed-off-by: Lajos Molnar <lajos@google.com>
Related-to-bug: 7093648