hwcontext: Perform usual initialisation on derived device contexts

The initialisation should be common.  For libmfx, it was previously
happening in the derivation function and this moves it out.  For VAAPI,
it fixes some failures when deriving from a DRM device because this
initialisation did not run.

libavutil/hwcontext.c

@@ -650,6 +650,10 @@ int av_hwdevice_ctx_create_derived(AVBufferRef **dst_ref_ptr,
     goto fail;
 
 done:
+    ret = av_hwdevice_ctx_init(dst_ref);
+    if (ret < 0)
+        goto fail;
+
     *dst_ref_ptr = dst_ref;
     return 0;
 

libavutil/hwcontext_qsv.c

@@ -1037,16 +1037,6 @@ static int qsv_device_derive_from_child(AVHWDeviceContext *ctx,
         goto fail;
     }
 
-    ret = qsv_device_init(ctx);
-    if (ret < 0)
-        goto fail;
-    if (s->handle_type != handle_type) {
-        av_log(ctx, AV_LOG_ERROR, "Error in child device handle setup: "
-               "type mismatch (%d != %d).\n", s->handle_type, handle_type);
-        err = AVERROR_UNKNOWN;
-        goto fail;
-    }
-
     return 0;
 
 fail: