Merge branch 'dtt-c-render' into py-package

Conflicts:
	overviewer.py

overviewer_core/src/overviewer.h

@@ -24,6 +24,10 @@
 #ifndef __OVERVIEWER_H_INCLUDED__
 #define __OVERVIEWER_H_INCLUDED__
 
+// increment this value if you've made a change to the c extesion
+// and want to force users to rebuild
+#define OVERVIEWER_EXTENSION_VERSION 2
+
 /* Python PIL, and numpy headers */
 #include <Python.h>
 #include <Imaging.h>