Start an example of basic lighting.

Draw a colored cube using indexed drawing.
Add a camera.
Add a directional light.
Merge pull request #21 from inmost-light/basic-lighting

Basic lighting
Remove duplicate matrix multiplication code.

The first call to `Matrix.multiply` forces the initialization of the
various default matrices. Their initialization code may call a
deprecated function, which in turn may cause a runtime exception. This
was the reason for the duplicate code in the first place. Making them
`lazy` seems to solve this problem.
Merge pull request #24 from inmost-light/matrix-mul-refactor

Remove duplicate matrix multiplication code.