Thumbnail size and rendering regressions

[timvandermeij]

Steps to reproduce:

1. Open http://kennemerlyceum.nl/attachments/article/4/Gymnasiumfolder.pdf
2. Open the sidebar with the thumbnail view
3. Notice that the second thumbnail is stretched in height and the third thumbnail has rendering issues (white background is missing for example).

Commits that caused this:

• Stretch caused by fe4ac86 (fixed by Fix thumbnail scaling regression for files with different page sizes (issue 5637) #5638).
• Rendering issues caused by ec311f3, i.e. PR Use white backdrop when possible. #3009.

[Snuffleupagus]

The referenced commit for the rendering issue is likely a red herring, further bisecting/triaging is needed.

Edit: Updated the description with the correct commit for the rendering regression.