Preserve transparency when loading from a file

Fix #7148 - again :)

Immediately after commiting I've noticed: Is this needed for `IMAGETYPE_GIF` as well?

@karlitschek backport?

cc @georgehrke @blackm @MorrisJobke @PVince81 

Edit: When using the file mentioned in #7148 it's still just black, that's because JCrop doesn't deal with transparency, but shows a black background itself:

My avatar with a transparent corner in jcrop:
![screenshot556](https://cloud.githubusercontent.com/assets/1410802/3888462/60a27fac-2200-11e4-99d5-bc6b21ea3ef8.png)

And when it's cropped:
![screenshot557](https://cloud.githubusercontent.com/assets/1410802/3888461/60a13890-2200-11e4-9163-205f99df832a.png)