Whole image not drawable when rotated

Tested in Pinta 0.6 & master on Linux, Pinta 1.0 on Windows.

While looking into bug #851378 I discovered this:

- Create an image of say 3000 x 500 (Any non-square surface will do, but the bug is more pronounced like this.)
- Have it zoomed to Window. (Anything that doesn't fill the entire screen will do.)
- Draw all over the image for a reference.
- Rotate the image 90 degrees
- Try to draw all over the image again.

You'll notice that you can only draw on the image in an area as long as the height of the image before rotating.

You can work around the issue by saving the image, closing it and then opening it again. It will then be fully drawable. (This might be a hint about where to start looking for the fix.)