For likely separate reasons, neither the non-OpenGL nor the OpenGL
code path produces what we actually want here (a line drawn in
'invert' mode if white is used, as happens on X11). In the non-OpenGL
case we get a black line, in the OpenGL case a white one. So let's use
grey and at least get the same effect in both cases.

Change-Id: Ifb587c6b46adb05e334a18add74352663cf7db9e