calculate gdk win32 monitor offset

[totaam]

So we can reliably convert between GDK coordinates (unsigned) and win32 coordinates (signed).
This is needed for #2943 and multi-monitors.
The offset values are in a private header, and are not exposed directly anywhere.

Here's how they are calculated:

  _gdk_offset_x = G_MININT;
  _gdk_offset_y = G_MININT;

  for (i = 0; i < data.monitors->len; i++)
    {
      GdkWin32Monitor *m;
      GdkRectangle rect;

      m = g_ptr_array_index (data.monitors, i);

      /* Calculate offset */
      gdk_monitor_get_geometry (GDK_MONITOR (m), &rect);
      _gdk_offset_x = MAX (_gdk_offset_x, -rect.x);
      _gdk_offset_y = MAX (_gdk_offset_y, -rect.y);
    }

Problems:

• ("making stuff up as we go") looks no different, so the offset will be G_MININT? How does it know to make stuff up?
• They filter out monitors... so if we don't do exactly the same thing, we will get a different offset
• This will need to be re-calculated whenever monitors are added or removed

Since the offset is global, perhaps we can just calculate it as long as we have at least one window on-screen?

[totaam]

Seeing the direction that GTK is taking (removing essential features because Wayland does not have them: #1977), it may be best to just replace GTK: #1995.