Without this we ask the AudioMerger to merge new buffers with N
channels with old buffers with M channels, which it can't do.
I'm not convinced this is the best solution but I think it will do
for now.
window size; we request images of the new size with set_video_container_size
but some old ones from the butler (I think) will arrive. Check that the
images are the right size before we try to construct a wxBitmap with them.
Should fix #1242.
