Display properties selects wrong color depth

Tested build: 0.4.15-dev-4557-g884356a06e1 (self-built)

Steps to reproduce:

• Boot either Live CD or Boot CD – use standard VGA PnP driver, no Guest Additions
• Open Display properties – screen mode should be 800x600x32
• Select screen mode 640x480x32 and press "Apply"
• Press "No" in the popup window once mode is changed to revert it
• Once it's back to 800x600x32, select mode 640x480
• Expected: 640x480 mode is selected, color depth is 32-bit
• Observed: 640x480 mode is selected, color depth is 24-bit

Bug is spotted by hpoussin. Restarting Display properties applet does allow to select it correctly, so I guess this is desk.cpl bug.

Debug log: 0.4.15-dev-4557-g884356a06e1.txt