Use ExifTool to get the dimensions of Flash files instead of calculating
it ourselves. Avoids copying third-party code.
Fixes a bug where Flash files with fractional dimensions (e.g. 607.6 x 756.6)
had their dimensions rounded down instead of rounded up.
Fixes another bug where Flash files could return negative dimensions.
This happened for two files:
* https://danbooru.donmai.us/media_assets/228662 (-179.2 x -339.2)
* https://danbooru.donmai.us/media_assets/228664 (-179.2 x -339.2)
Now we round these up to 1x1. This is still wrong, but it's less wrong than before.