Especially in the initial phase before calling Run/RunGame, the
window position is not reliable and then getting the device scale
factor does not make sense based on the window position. Avoid
using the window position, and instead use the glfw.Monitor in
this situation.
On Windows, the window could be maximized even when the window was
not resizable. This behavior is confusing. Forbid it so that the
behavior will be clearer.
This change changes the behavior of WindowPosition /
SetWindowPosition. The window position is now a relative position
and the origin position is the left-upper of the current monitor.
Fixes#1115
