fix pdf previews staying low resolution

I noticed this problem a while ago when browsing pdfs. Notice the broken preview of the fourth pdf:

Browsing online, the issue is also briefly mentioned here as unsolved: https://askubuntu.com/questions/1340030/how-to-fix-tumbler-random-image-thumbnail-sizes-in-thunar

It turns out the culprit is that tumbler always prefers embedded thumbnails when a pdf contains one. I added a check to use the embedded thumbnail only if its resolution suffices for the request. As I had to create an account for this issue and cannot fork the repository yet, I pushed my fork to github: https://github.com/v4hn/xfce-tumbler/tree/pr-resolution-beyond-embedded-pdf

Please consider this issue a Merge Request to solve the problem upstream.