Inconsistent double tap to zoom behavior

[gaearon]

Thanks for a great library.

I've noticed that the "double tap to zoom" behavior was inconsistent between platforms — in particular, the manual calculations in the Android part were zooming in way too much compared to the iOS version.

I reimplemented these parts to be consistent in our fork bluesky-social/social-app#1482, so wanted to share in case someone finds them useful.