macOS: Fix coordination error in multi screen environment.
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Fix bug that gdk_monitor_get_workarea() always return workarea for main screen.
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Fix bug that gdk_monitor_get_geometry() return wrong gemotry that is NOT based on GdkScreen coordination. This bug also causes gdk_display_get_monitor_at_point() can not find correct monitor at specified point.
Fixes #1593 (closed)