0.64 Restore from maximized mode (Hide title bar) issue

[PinkPanther]

Hi Pierre!

Steps to reproduce:
- start XnViewMP
- maximize it
- press Ctrl+F11 to hide tile bar
- XnViewMP is now maximized without the title bar
- press Ctrl+F11 to show the tile bar
- XnViewMP resizes down to a small window

- maximize it once again
- press Ctrl+F11 to hide tile bar
- XnViewMP is now maximized without the title bar
- press Ctrl+F11 to show the tile bar
- now XnViewMP stays in maximized mode

This happens to me only in "Hide tile bar" mode. System is Windows 8.1 x64. Tested in 0.64 and 0.61.

Thanks!