x11: Keep track of the exact size in X pixels of windows

x11: Keep track of the exact size in X pixels of windows

Keep track of the exact size of X windows in underlying pixels; we
generally use the scaled size instead, but to properly handle the GL
viewport for windows that aren't a multiple of window_scale,
we need to know the real size.

https://bugzilla.gnome.org/show_bug.cgi?id=739750