Blends the overlay rectangles in this on top of the raw video data contained in video_buf.
The data in video_buf must be writable and mapped appropriately.
Since video_buf data is read and will be modified, it ought be mapped with flag GST_MAP_READWRITE.
| this | |
| video_buf |
a Frame containing raw video data in a supported format. It should be mapped using GST_MAP_READWRITE |