The wallpaper gets scaled down after restart, and does not fit the bigger resolution monitor

 * Cinnamon version: 3.4.6
 * Distribution: Mint 18.2
 * 64 bit
 * inxi -G
Graphics:  Card: Intel Sky Lake Integrated Graphics
           Display Server: X.Org 1.18.4 drivers: intel (unloaded: fbdev,vesa)
           Resolution: 1366x768@60.00hz, 1920x1080@60.00hz
           GLX Renderer: Mesa DRI Intel HD Graphics 520 (Skylake GT2) GLX Version: 3.0 Mesa 17.0.7

Issue

The wallpaper gets scaled down after restart, and does not fit the (secondary) bigger resolution monitor.

Steps to reproduce

• Set a 2560x1600 px image as a centered desktop wallpaper. It covers all the screen area for both 1366x768 px and 1920x1080 px screens.
• Restart the computer.
• The wallpaper seems to be scaled down to 1366x768 px resolution and covers the smaller (primary) monitor all right, but on the larger resolution monitor the image is smaller now and does not cover all the screen, there are black margins.

Expected behaviour

• The wallpaper is displayed the same way after restarting the computer as it was right after I set it up.

Other information
After restart, if I go to wallpaper settings and click on another image and right back to the original one, the wallpapers are once again set up properly.

Start by reproducing the multi-monitor wallpaper setup and restart sequence described in the issue, then trace where the selected wallpaper layout and monitor dimensions are persisted and restored. Verify the behavior by switching images as described and confirm that the original centered wallpaper again fills both monitors after restart.