pywrap: Enable default NULL callback typemap.

It was disabled by mistake while callback proxy was reimplemented.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
pywrap: Progress callback proxy check for Py_None.

If a wrapped function is called with callback parameter
set to None, i.e.

  callback = None
  loaders.Load("image_path/image", callback);

The progress callback fails because Py_None is not executable.

Fix this by checking for Py_None and skipping the callback if passed.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Fix typos in GP_TempAlloc() documentation.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Initial MultiTone and Sepia filters.

Not finished yet.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>