Initial window positioning fix for WinGL & Linux (fixes #1414) - DEPENDS ON OPENTK CHANGE

The window is created before being resized on WinGL and Linux, which means it's centered based on the wrong initial size. This pull request makes sure the window is properly centered just before the main loop starts.

This pull requests **depends on an OpenTK change**: https://github.com/dellis1972/opentk/pull/1 so we should make sure the binaries are updated before merging otherwise it won't work (it won't break anything but the centering won't work as OpenTK's ClientRectangle setter used to ignore the position)