Modelled after the xfree86 code. Call miDCInitialize to init the SW rendering
engine, then take the pointers, store it in a xnest-local variable, and put
the xnest-specific sprite funcs in place. In the xnest sprite funcs, call
through to the mi sprite funcs after doing xnest-specific stuff.
Xnest was half-using midispcur and doing a bad job of it. Replace all of
that code with mipointer which does a lot of the work.
Support DDXen which don't provide GetWindowPixmap, or which return NULL for
the root pixmap.
