euler: Fix the math for the matrix conversion

We are using a left-handed sign convention, to ensure that creating a
matrix through graphene_euler_t, graphene_quaternion_t, and using the
angle-axis notation results in the same rotation matrix.

We need to document the exact nature of our transformation, including
the matrices used to compute the resulting row vectors.
tests/matrix: Verify that all rotations are idempotent

Rotating through all the various API we provide should produce the same
results.