Support for HiDPI screens.

[knokelmaat]

Hi!

Gtk supports HiDPI scaling at integer scaling factors. However, for each game, the cover is saved at the LoDPI resolution (300x200). When scaling is enabled (e.g. in GNOME using display settings) the UI is cleanly rendered at double the size, but the cover images are blurry, as it is the low resolution image that is being upscaled.

A solution to this would be to save the images in the original resolution (what steam does) and apply scaling to a factor of 300 x 200 as needed when loading the covers. I implemented an initial version of this and it seems to work correctly.

The only downside is that a little more storage space is used (the original image files are stored in the data folder instead of the already scaled ones).

It is probably easier to just see it in code, I'll make a copy of the repo and show you my changes :).

[knokelmaat]

Original, blurry at 200% scaling.

New, sharp at 200% scaling