Issue 1504: changed how `draw` handles alpha in `markerfacecolor`

changed how `draw` handles alpha in `markerfacecolor` so that the set  value respected.

At least in the Agg backend, the alpha value of the face color passed into draw_markers (in _backend_agg.cpp) is overridden by the value in gc_obj.  It is not clear to me this in the correct behavior, but these changes fix the problem with minimal changes