Previously, the new mode was added at the head of the list. This caused the
positional correspondence between modes and the XMesaVisuals array to be off
by one. The net result was GLX clients failing when they tried to use the
last GLX mode/visual.
We still have the problem of DRI drivers not being able to use the extra
mode/visual introduced by __glXCreateARGBConfig(). glXCreateContext fails
with BadAlloc if it's attempted. This is also the source of the often-
seen warning "libGL warning: 3D driver claims to not support visual xxx"
Look into fixing that someday...