command: add property returning detected hwdec API

This is somewhat imperfect, because detection of hw decoding APIs is
mostly done on demand, and often avoided if not necessary. (For example,
we know very well that there are no hw decoders for certain codecs.)

This also requires every hwdec backend to identify itself (see hwdec.h
changes).

video/vaapi.c

@@ -130,6 +130,7 @@ struct mp_vaapi_ctx *va_initialize(VADisplay *display, struct mp_log *plog)
         .log = talloc_steal(res, log),
         .display = display,
         .hwctx = {
+            .type = HWDEC_VAAPI,
             .priv = res,
             .vaapi_ctx = res,
             .download_image = ctx_download_image,