stage: re-implement minimal paint() method to respect Z order

Without a paint() implementation in clutter-stage, the function
from clutter-group is used.  That class has its own child list,
but attempts to use sort_depth_order, which is empty in this case.

This provides a partial fix by replacing a minimal paint(), see:

https://bugzilla.gnome.org/show_bug.cgi?id=711645