Add SDL frontend.
Extend math library.
Add color to the sphere defined in main.cpp.
Add color to the sphere defined in sdl_main.cpp.
Properly support colors.
Replace hardcoded light with one acquired at runtime.
Pass a light to the raytracer in main.cpp.
Pass a light to the raytracer in sdl_main.cpp.
Add vec3_zero() to mathlib.
Fix incorrect shading with multiple lights.
Add few lights to the SDL frontend to make it prettier.
Make it possible to change the rotation speed of lights with left and right arrow.
If the calculated light intensity for a given pixel is negative skip color mixing instead of just zeroing the intensity.
Prune unused local variables in sphere_intersect().