wx 2.9 compile fix for INITIALIZE_FAST_GRAPHICS

NOTES
* the entire wxDC code was reorganized with wx 2.9
* need to include wx/gtk/dcclient.h, which isn't installed (so not part of externpro)
* wxplotctrl needs an additional include directory to wx source
* haven't verified this code actually still works with wx 2.9, just that it now compiles