hidpi: Background is blurry in HiDPI mode.

 * slick-greeter version - 1.2.2
 * Distribution - Arch
 * Graphics hardware *and* driver used - Intel HD620
 * 32 or 64 bit - 64 bit

Issue
I set this image as my background image in slick-greeter. My laptop has a 14" 2560x1440 screen, so I typically need HiDPI options for things to be a usable size. Hence, I set the following in my slick-greeter.conf:

draw-grid=false
enable-hidpi=on
xft-dpi=180

The result was a crisp greeter with appropriately-sized text, but a weirdly-scaled and blurry background. Here is the result in HiDPI mode (as configured above), and here is the result in "normal" mode. Notice how much higher-quality the greeter wallpaper is in the non-HiDPI version.

Steps to reproduce
Just download the image above and run the greeter with this minimal configuration:

[Greeter]
background=/usr/share/backgrounds/mountains-3.jpg
draw-grid=false
enable-hidpi=off

Expected behaviour
I wouldn't expect the background to be scaled in any way (just UI elements), but it is.

Reproduce the issue using the supplied mountains-3.jpg and minimal slick-greeter.conf, comparing enable-hidpi on and off at 2560x1440. Start by tracing the greeter's background-loading and HiDPI scaling path; done means the background remains sharp while UI elements retain the configured HiDPI sizing.