feedInput
Push one frame into input index; each output frame is handed to onOutput as it comes out, before the graph is asked for the next one. Closes frame (the graph keeps its own reference). Output frames are valid only for the duration of the callback; Frame.copy to keep one.
onOutput runs with this graph's lock released, so it may call back into the graph. Two things it can do end the call: throw, and the throw comes out of this call, or close the graph, and the call returns without asking the graph for anything more. Either one stops a filter that never stops producing, such as tpad=stop=-1 once its input has ended.
Return
how many frames came out, and whether the graph now waits for a particular input
Throws
when input index was flushed