Take machine's screen scale factor into account

[itsthejb]

Hey Jan,

Handy little lib, but it seems the image rendering is not device independent: on a retina-screened device this will double the size of the destination image, on a non-retina screen it will remain 1:1. This would suggest that it's drawing the source image into a context with a scale factor to 2 for retina screens, and 1 otherwise.

Unfortunately Xcode validates icon sizes when using Asset Catalogues so this results in failed builds.

No time to do a PR, I'm afraid 😢