Work around Xlib 64-bit "specialness"

When putting 32-bit properties into longs on 64-bit architectures,
XGetWindowProperty assumes the values are supposed to be signed, and
so it sign-extends values greater than 0x7fffffff. So if they *aren't*
supposed to be signed, we need to chop off the high bits ourselves.

(Most CARDINAL-valued properties only end up using small values
anyway, so it doesn't matter, but _NET_WM_WINDOW_OPACITY uses the full
range, and so was previously failing on 64-bit machines.)

https://bugzilla.gnome.org/show_bug.cgi?id=605678