Scroll - Fixing buggy scroll behaviour on less than min-zoom.

[tdekoning]

This pull request fixes issue #41 .

To reproduce:

• Go to "Advanced demo"
• Open an image
• Press the "rotate & crop" icon on the top right
• Make the image about half of the size of the white rectangle
• See that panning, rotating and zooming still works properly

[alexvasilkov]

Sorry, I already fixed it on the weekend, but forgot to push changes (d2ad654c648e6899cc455d44ae4d8b4cb2cd8576). Anyway, thanks for the effort!