X11: Fix no window position events during resize

A window resize action that also resulting in the window being moved did
not emit any window positions events, as the position of real
ConfigureNotify events was ignored.  The real events use parent
coordinates instead of root coordinates so this adds parent tracking and
conditional translation.

Fixes #1613.

src/x11_platform.h

@@ -186,6 +186,7 @@ typedef struct _GLFWwindowX11
 {
     Colormap        colormap;
     Window          handle;
+    Window          parent;
     XIC             ic;
 
     GLFWbool        overrideRedirect;