core: Fix thumbnails with transparent backgrounds, fixes #379 #380
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before
![image](https://user-images.githubusercontent.com/1006268/31275323-1aecd8f4-aa97-11e7-9a29-4f88849b6481.png)
after
![image](https://user-images.githubusercontent.com/1006268/31275311-0d277f4e-aa97-11e7-88f5-5c1b6bbb7d8f.png)