FFmpeg uses a fraction of integers to indicate the frame rate, whereas Blender uses `int / float`. When a custom frame rate is used with non-integer base, the FPS and Base settings were multiplied with 100000 before passing to FFmpeg as `int`. This could overflow when a high enough FPS setting was used, which is the case when importing a video of almost-but-not-quite-integer frame rate into the VSE. The overflow caused FFmpeg to return an error "The encoder timebase is not set", which is rather cryptic for users. The new solution is to take the max int and divide that by the frame rate, and use that ratio to pass to FFmpeg. This won't overflow, and thus allows exporting arbitrary frame rates.