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.


Surround all operators (`*`, `/`, etc.) by spaces: `m_height * m_height`