lavc: external hardware frame pool initialization

This adds a new API, which allows the API user to query the required
AVHWFramesContext parameters. This also reduces code duplication across
the hwaccels by introducing ff_decode_get_hw_frames_ctx(), which uses
the new API function. It takes care of initializing the hw_frames_ctx
if needed, and does additional error handling and API usage checking.

Support for VDA and Cuvid missing.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
