a5b7cf9b5f
The root cause seems to be an assumption in [generate_video_frame()](https://developer.blender.org/diffusion/B/browse/master/source/blender/blenkernel/intern/writeffmpeg.c) that we're always using 4 bytes per pixel. This is not true when using QTRLE in RGB mode, because that uses the RGB24 pixel format (so 3 bytes per pixel). Just updating the `linesize` property doesn't fix it though, but just creates a crash somewhere else. This at least fixes the crash by always forcing RGBA to be written, even when the user selects RGB.