With the VPX decoders it's not simple to use downstream buffer pool,
because we don't know the image size and alignment when buffers get
allocated. We can though use GstAllocator (for downstream, or the system
allocator) to avoid a copy before pushing if downstream supports
GstVideoMeta. This would still cause a copy for sink that requires
specialized memory and does not have a GstAllocator for that, though
it will greatly improve performance for sink like glimagesink and
cluttersink. To avoid allocating for every buffer, we also use a
internal buffer pool.
https://bugzilla.gnome.org/show_bug.cgi?id=745372
This adds an automatic mode to the threads property of vpxdec in order to
use as many threads as there is CPU on the platform. This brings back
GStreamer VPX decoding performance closer to what is achieved by other
players, including Chromium.
https://bugzilla.gnome.org/show_bug.cgi?id=758195
They are reported properly by libvpx if the correct struct members are used.
This also fixes handling of resolution changes without input caps changes.
https://bugzilla.gnome.org/show_bug.cgi?id=719359
