added support for vertex tangents in renderer::StaticTessellation and renderer::MeshObject.
optionally compute smooth vertex tangents after reading mesh objects from disk.

to compute smooth vertex tangents for an object or a set of objects, add a "compute_smooth_tangents" parameter to the <object> element of the project file and set it to a Perl regular expression matching the object(s) for which you want to compute smooth vertex tangents:

    <object name="geometry" model="mesh_object">
        <parameter name="filename" value="bunny.obj" />
        <parameter name="compute_smooth_tangents" value="ear.*" />
    </object>
use smooth vertex tangents when available, plus many refactorings and cleanups.