vtenc: support VT buffer pool
Submitted by Ilya Konstantinov
Link to original bug (#748503)
Description
VT provides VTCompressionSessionGetPixelBufferPool to get a CVPixelBufferPool. Using CVPixelBuffers from this pool will avoid memcpy into hardware buffers.
For this, we should implement:
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GstCoreVideoMemory (bug 747216)
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GstCoreVideoBufferPool extending GstBufferPool with a custom alloc_buffer
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vtenc's propose_allocation with a single GstCoreVideoBufferPool allocation pool