Revert "clip-boxes: Drop too-early return"

This reverts commit cb871c6c69.

The original code is badly, but removing the early return leads to
crashes.  This code will need a lot more attention to get right, for now
at least go back to the original behavior.

Fixes: https://gitlab.freedesktop.org/cairo/cairo/issues/358#note_125270

src/cairo-clip-boxes.c

@@ -524,6 +524,7 @@ _cairo_clip_reduce_to_boxes (cairo_clip_t *clip)
     cairo_clip_path_t *clip_path;
     cairo_status_t status;
 
+	return clip;
     if (clip->path == NULL)
 	return clip;