utils/libav: add get_buffer2 implementation

Similar to Dav1dPicAllocator, this allocates from DMA-visible GPU memory. Especially useful on platforms that don't support arbitrary host pointer imports.

As an aside, also improve the handling of single-allocation AVFrames in general, which is a common-ish use case. As usual, some stupid gymnastics are performed to try and detect our mapped buffers when given arbitrary AVFrames.

Make plplay use this, for testing purposes.