hwcontext: Improve allocation in derived contexts

Use the flags argument of av_hwframe_ctx_create_derived() to pass the
mapping flags which will be used on allocation.  Also, set the format
and hardware context on the allocated frame automatically - the user
should not be required to do this themselves.

libavutil/hwcontext_internal.h

@@ -121,6 +121,11 @@ struct AVHWFramesInternal {
      * context it was derived from.
      */
     AVBufferRef *source_frames;
+    /**
+     * Flags to apply to the mapping from the source to the derived
+     * frame context when trying to allocate in the derived context.
+     */
+    int source_allocation_map_flags;
 };
 
 typedef struct HWMapDescriptor {