Add de265_push_end_of_frame function.

Add de265_push_end_of_frame function to indicate that de265_push_data has just received data until the end of a frame. All data pending at the decoder input will be pushed into the decoder and the decoded picture is pushed to the output queue.

This function is needed when feeding individual frames to the decoder. (And jumping in the bit stream.)
This function forces the decoded picture out at the frame end - and it is not needed to feed the first slice of the next frame to the decoder to get the current frame out.