Zoom is doubled

[Jakobud]

Whenever I bring in an image the zoom is double what it should be. I have to set the zoom of the image to 50% in order to see the image at it's actually 100% size.

[rocketnik]

Fixed in #4

In this PR, the image's size in pt instead of px is fetched (when available) and the created window's size matches the image's pt size.

Unfortunately, if you create a screenshot from an iOS Simulator, the screenshot does not contain the correct scale factor for the image and thus is displayed too large, when opened. I have no idea, what to do about it.

There is a workaround however: there are more zoom levels supported, so that it should be possible to scale the image to the desired resolution.