Tag imported TrueType layers correctly.

As noted in #1552, TrueType files were importing with the background layer incorrectly tagged as a foreground layer.

SplineCharCreate is not a smart function and just creates the specified number of layers. In all other cases in FontForge (as far as I can tell), the function constructing the SplineChar is responsible for setting the layer attributes. The background tag operates separate of the layer index, and, depending upon the file format, multiple layers can have background status.