Blender Git Loki

Fix T53058: Crash when rendering to Quicktime RLE codec

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.