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:

1. GstCoreVideoMemory (bug 747216)

2. GstCoreVideoBufferPool extending GstBufferPool with a custom alloc_buffer

3. vtenc's propose_allocation with a single GstCoreVideoBufferPool allocation pool

Depends on

• Bug 747216