Improve mouse selection for windows with padding

Moving the mouse into the padding area now acts as if the mouse is over
the nearest cell. Fixes #430

Note that this only applies to padding defined via the
window_padding_width option, not any leftover space from the window size
not being an exact multiple of the cell size.

kitty/state.c

@@ -339,6 +339,7 @@ PYWRAP1(set_options) {
     S(cursor_stop_blinking_after, PyFloat_AsDouble);
     S(background_opacity, PyFloat_AsDouble);
     S(inactive_text_alpha, PyFloat_AsDouble);
+    S(window_padding_width, PyFloat_AsDouble);
     S(cursor_shape, PyLong_AsLong);
     S(url_style, PyLong_AsUnsignedLong);
     S(tab_bar_edge, PyLong_AsLong);