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 AVHWAccel.priv_data_size to frame_priv_data_size

This describes more accurately what this field is for.
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 replced with a new one.
vda: use hwaccel private data for internal bitstream buffer
lavc: add a new vda hwaccel
avconv: support VDA hwaccel
vda: small fixes
vda: Do not trigger the f.buf[0] assert

VDA fills internally the buffer.
vda: Fill the missing dictionary key

Otherwise it won't initialize the decoder.
vda: map return values
vda: Unbroken
vda: Fix last frame handling

Otherwise it would segfault.
hwaccel: propagate the private context