KMPlayer has an option. But did not seem to be working.
says that the latest version seems to do it.
says ffmpeg can do it.
the -y option means overwrite without asking.
But simplest is Virtualdub,
showed the way using File -> Export -> Image Sequence.
In Virtualdubmod, the menu item is File -> Save image sequence
For exporting the video as an image sequence for tgawarper, I need not bother about aspect ratio - tgawarper will warp it anyway in the correct manner if the frame contains a fisheye view with no black edges.