lavc: Add an internal wrapper around get_format()

It will be useful in the following commits.
lavc: set AVCodecContext.hwaccel in ff_get_format()

This way each decoder does not have to do the same thing manually.
lavc: document which parts of AVHWAccel are private.
hwaccel: Rename priv_data_size to frame_priv_data_size

This describes more accurately what this field is for.
hwaccel: Support specific frame allocators

It would reduce the boilerplate code users have to write.
lavc: Add hwaccel private data and init/uninit callbacks
lavu: Add a pixel format for new VDA hwaccel

The current hwaccel is broken and cannot be fixed in a compatible
way. It will be deprecated and replaced with a new one.
vda: use hwaccel private data for internal bitstream buffer
lavc: Add new VDA hwaccel

It leverages the new hwaccel 1.2 features:

- get_buffer2 is never called
- the internal context is automatically initialized/deinitialized

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
avconv: Support VDA hwaccel

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>