lavfi: generic hardware surface upload and download filters

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/avfilter.h

@@ -300,6 +300,15 @@ struct AVFilterContext {
      * An opaque struct for libavfilter internal use.
      */
     AVFilterInternal *internal;
+
+    /**
+     * For filters which will create hardware frames, sets the device the
+     * filter should create them in.  All other filters will ignore this field:
+     * in particular, a filter which consumes or processes hardware frames will
+     * instead use the hw_frames_ctx field in AVFilterLink to carry the
+     * hardware context information.
+     */
+    AVBufferRef *hw_device_ctx;
 };
 
 /**