more unit tests for the quaternion to 3×3 matrix conversion.

with the unit tests.

Euler angles to a rotation matrix. Also fix quat::rotate() which was not
building the correct quaternion.

mathematical writing, and add static constructors to create quaternions
from a rotation.

code for all matrices.

replace the well known DeBruijn sequence with a custom magic value to spare
one binary operation.

"shader" source directory since we have "gpu".