generate: choose correct framerate for GIF compositing

[Montspy]

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

1. The ImageBuilder should be able to detect the framerate of GIF layers and using that
2. We could add a "force framerate to xx fps" box in the create/edit collection UI for animated collections