generate: choose correct framerate for GIF compositing #106
Description
Currently ffmpeg defaults to 25fps when importing GIFs for compositing.
It's causing frame-pacing issues for GIFs with frame delays not multiples of 1/25s
- The ImageBuilder should be able to detect the framerate of GIF layers and using that
- We could add a "force framerate to xx fps" box in the create/edit collection UI for animated collections