In the current implementation, images with some other scaling than 100%
are only loaded when an image is available that exactly fits to the
desired zoom level. In all other cases, the 100% image is used as a
fallback.

With this change, the directly fitting or next larger available
image of 150% or 200% is used. Only if none of them exists, the 100%
image is used as a fallback. This improves the quality of images when a
display scaling of more than 100% is used and no image is provided for
exactly that scaling. To simplify the changes, DPIUtil methods returning
multiple values were changed to not use parameters to pass returns
values and some unnecessary case distinctions have been cleaned up.

The change is applied to the Windows and Linux/GTK implementations. Mac
only requires handling for 100%/200% and already has that implemented.