Since there can be multiple backends for a single API (vaapi can use GLX
or EGL), not logging the exact backend name is annoying. So add it. At
the same time, there is no need to duplicate the name as used by the
--hwdec options, so replace it with using the numeric hwdec API ID.
Surfaces used by hardware decoding formats can be mapped exactly like a
specific software pixel format, e.g. RGBA or NV12. p->image_params is
supposed to be set to this format, but it wasn't.
(How did this ever work?)
Also, setting params->imgfmt in the hwdec interop drivers is pointless
and redundant. (Change them to asserts, because why not.)
