Preserve transparency when window-crop is used

Use `-compose Copy` to preserve transparency when window-crop is used on PNG's. This is only being added to the `window-crop` mode and not also `window-crop-fixed` because the latter doesn't appear to be used as frequently and it is typically coupled with a `--fill` option which might complicate things further.

https://wikia-inc.atlassian.net/browse/PLATFORM-604

/cc @nmonterroso