added a fast resize function

Added resize_half() and resize_half_wrap() functions to composite.c and
overviewer.h, to replace the call to PIL's resize function made by
tileset.py. Also added "resize_half" to COverviewerMethods in main.c, so
it can be called from Python. Should increase performance by 10 to 20%
for the entire program.

overviewer_core/src/overviewer.h

@@ -69,6 +69,8 @@ PyObject *draw_triangle(PyObject *dest, int inclusive,
                         int x2, int y2,
                         unsigned char r2, unsigned char g2, unsigned char b2,
                         int tux, int tuy, int *touchups, unsigned int num_touchups);
+PyObject *resize_half(PyObject *dest, PyObject *src);
+PyObject *resize_half_wrap(PyObject *self, PyObject *args);
 
 /* forward declaration of RenderMode object */
 typedef struct _RenderMode RenderMode;