So a hwaccel can access avctx->hwaccel in init for whatever reason. This
is for the new d3d hwaccel API. We could create separate entrypoints for
each of the 3 hwaccel types (dxva2, d3d11va, new d3d11va), but this
seems nicer.

Merges Libav commit bd747b92.
Signed-off-by: Diego Biurrun <diego@biurrun.de>