gdksurface-win32.c: Disable AeroSnap for surface if not resizable

Hi,

From the commit message:

AeroSnap essentially resizes the surface by all means, so if a surface is not resizable, just bail out of the operation, even though it is triggered by a move operation.

This matches what is done in other Windows applications.

This will also be for gtk-4-2.

I will also work on a GTK-3.x version of this shortly, if I could find a way to check whether the GdkWindow is actually resizable :S.

With blessings, thank you!