Remove the tile buffer

This is helping only very slightly, and it's a bunch of extra complexity. Also, the buffering can use an astronomical amount of memory on large images. I think we're better off without it.
Remove histogramimage.cpp

Not the right place for color model code
Remove packWavelength() call for now
Move random variable support to a separate Sampler object
More sampler refactoring
Color rendering, line clipping
Example with multiple lights
Implementation for blackbody variables