GLX returns a wrong value if the y coords are inverted. Most other
programs don't even ask for this (gnome-shell for example) and just
assumes "true" (and this works because most relevant X servers work
like this). We make this more robust and assume true only if the
returned value is GLX_DONT_CARE (-1).

Change-Id: I4800b3364fd00f5f4a8f5a459472bfa8d97827ba