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This CL teaches VaVDA to use client VASurfaceIDs to decode onto, saving
a buffer copy and removing a costly blit (DownloadFromSurface) on the
GPU Main thread.  Three groups of changes:

1. In AssignPictureBuffers(): if |vaapi_picture_factory_| Create()s a
 VaapiPicture with a VASurfaceID, we use those to decode onto and set a new
 flag: |decode_using_client_picture_buffers_|

2. When the decoder calls CreateVASurface(), instead of giving it the first
 |available_va_surfaces_|, we need to figure out the first VASurfaceID in
 this |available_va_surfaces_| such that the corresponding VaapiPicture in
 |pictures_| is available (i.e. in |available_picture_buffers_|).
 Reason: libva holds on to some VASurfaceIDs, there's no simple one-to-one
 correspondence like on ToT.

3. When we're ready to OutputPicture() to |client_|, instead of using the first
 |available_picture_buffers_|, we find the corresponding one for the
 passed |va_surface_id| (we could search all over |pictures_| but there's
 always less |available_picture_buffers_|.

Some other notable developments:

- s/output_buffers_/available_picture_buffers_/
- OutputPicture() loses a parameter.
- |decode_using_client_picture_buffers_| is still false for e.g. VP9 Profile 2
 or the ImportBufferForPicture() path.
- Pictures is made a base::small_map. (Originally from crrev.com/c/988512).

Bug: 822346, 717265
Test: v_d_a_unittest vp8/vp9/h264 passing on eve, running crosvideo changing
resolutions for a long time. No hickups, no dropped frames.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I3d5a4d3c83002c8d0977702c9b97ffec52b1db23
Reviewed-on: https://chromium-review.googlesource.com/1021675Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568880}