refactoring and renaming things.
created the renderer::PixelContext class identifying the pixel being rendered throughout the rendering pipeline.
tweaked assertions.
the tile bounding box passed to renderer::IPixelRenderer::render_pixel() is now using signed integer coordinates.
foundation::knn::Query can now query the k nearest neighbors within a given maximum distance.
appleseed.package.py 2.3.2: fixup the Qt frameworks on Mac OS X.
first mostly working implementation of sppm.