Add ability to define custom bokeh shapes with a texture

Hi, I think I have implemented the issue:
https://github.com/appleseedhq/appleseed/issues/210

 - Modify ThinLensCamera class, add an ImageImportanceSampler to generate_ray
 - Add a test scene and a result.


What about only allowing texture instances to be bound? Like it's done in e.g. the lat-long environment EDF:
https://github.com/appleseedhq/appleseed/blob/master/src/appleseed/renderer/modeling/environmentedf/latlongmapenvironmentedf.cpp#L413