The check for undo-depth increment/decrement assumed a newly loaded
window manager would have a different pointer.
This broke bl_animation_fcurves test indirectly,
the change to undo-depth caused the redo panel to attempt to popup
in background mode - which isn't supported.
Now the pointer is unchanged, the undo-depth is assumed to match
the value used when calling the operator.
The undo-depth is now properly maintained between file loads,
which is an improvement on the original behavior which reset it.
9dfc81ccf1