Replace the gfx-based implementations of the Epicardium display API
calls with ctx-based ones that try to mimick the old behavior as much as
possible.  In the long run, these functions are just meant for backwards
compatibility while new code should interface with ctx directly (once
this becomes possible).

The implementations in this changeset are closely based on pippin's
original Pycardium display API reimplementation.

Co-developed-by: Øyvind Kolås <pippin@gimp.org>