Native window position is not set

[Meakk]

Describe the bug

The default position of native windows is defined in VTK, and seems to be:

• 50,50 for Cocoa
• 0,0 for Windows and Linux (to confirm)

Moreover, Y=0 corresponds to the bottom of the screen for Cocoa, so the Y position must be reworked in VTK to flip it using the height of the screen.

Also, maybe a setPosition public API could be useful.

[Meakk]

Related: #409

[mwestphal]

Linux:

• Awesome (floating) : Window is not positioned correctly, half of it is below the screen
• Xfce: Window is positioned perfectly, top left, without hiding the "Xfce bar"
• I3 (floating): Window is positioned center right of the screen, perfectly acceptable
• Openbox (floating) : Window is not positioned correctly, half of it is below/between screens

WIndows:

• Window is on 0,0

[mwestphal]

Fixed by #444