Lights are now intersectable.

A Light must implement an ::intersect() method. raytrace() checks
for such intersections and returns the light color in this case.
Speed up K-d tree intersection.

Add BBox::intersectWall() method, which is used in intersectKD().
The speedup is as much as 30% (see http://raytracing-bg.net/?q=node/68
for thorough explanation).
Make the Rect light's output dependant on the lamp size.

This, making the lamp larger, without changing its color, would now make
the scene brighter. This formulation is now consistent with the path tracing algorithm.
Two scenes, to be used in demonstrations of the path tracing GI algorithm.

smallpt.trinity:     recreation of Kevin Beacon's 'smallpt' path-tracer example.
cornell_box.trinity: a Cornell Box scene, converted in our format by
                     Ivaylo Dankolov of the RTBG2011-2012 course.
Small fixes to cornell_box.trinity.
From Lecture 12: implementation of the path tracing algorithm.

Not polished right now, and needs lots of comments.
Remove multiplication with cos(N, lightDir) in pathtrace's explicit light sampling.

This calculation is already done in the BRDF, so in result the picture is overcompensated
for the cosine law, and is a bit too dark.
Some rearrangement in the terms of the explicit light sampling.
Add comments to pathtrace(). Change the GI test scenes a bit.
Disable stereoscopy for meshes.trinity
Add CXXPTL for multithreading.
Multithreaded with CXXPTL
Raw code from lecture 13.
The interactive code is in mainloop(), and the interactivity is enabled
for scene testmesh.trinity.
Add cxxptl_sdl.cpp/h to project files.
Use -O3 -ffast-math compiler flags.
Use -O3 -ffast-math compiler flags on Linux, too.
Data files for easy project #4 (Beer's law)

When rendering the scene, the parser complains about
missing properties (insideColor, absorption, etc.).

This is because these are te properties you have to implement.
Display statistics about the performance in interactive mode (average FPS).
Don't display individual buckets in interactive mode.

The whole frame is displayed with displayVFB() anyway; extra drawing
only slows things down.
Don't wait for ESC after rendering in interactive mode.