GDK-Win32: Fix AeroSnap indicator and positioning

GDK-Win32: Fix AeroSnap indicator and positioning

Ensure that we take the DPI scaling into account so that surfaces will
be placed at their correct positions upon an AeroSnap operation on HiDPI
displays.

Also, use the X coordinate of the surface as-is during snap up so that
we do not inadvertently move the surface to the very left.  Also fix the
AeroSnap indicator drawing for snap up so that it is drawn at the
correct places.

Since we are updating these functions, make the old GdkWindow-era
variable names to match better the names we use nowadays.