Interplay MVE movies have a SEND_BUFFER operation. Only after this
command does the current decoding buffer get displayed. This is required
for the other frame formats. They are fixed-size and can't always encode
a full frame worth of pixeldata.

This code prevents half-finished frames from being emitted.
Signed-off-by: Hein-Pieter van Braam <hp@tmm.cx>