Added scenes/sphere.{hpp,cpp} using the cube-to-sphere
approach from nrz.cpp, but with corrected math: vertices
are simply normalized to project onto the unit sphere
(the original used a broken formula with p=50.0 as an
exponent).
The sphere uses indexed geometry with position, normal,
and UV attributes, plus a simple diffuse lighting shader.
Press 1/2 to switch between cube and sphere scenes.
Updated .gitignore to exclude generated PNG screenshots.
portersky