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/main.c

@@ -26,6 +26,9 @@ static PyMethodDef COverviewerMethods[] = {
     {"alpha_over", alpha_over_wrap, METH_VARARGS,
      "alpha over composite function"},
     
+    {"resize_half", resize_half_wrap, METH_VARARGS,
+     "downscale image to half size"},
+    
     {"render_loop", chunk_render, METH_VARARGS,
      "Renders stuffs"},