lavc: Add hardware config metadata for decoders supporting hardware output

lavc: Add hardware config metadata for decoders supporting hardware output

This includes a pointer to the associated hwaccel for decoders using
hwaccels - these will be used later to implement the hwaccel setup
without needing a global list.

Also added is a new file listing all hwaccels as external declarations -
this will be used later to generate the hwaccel list at configure time.