Current transformation matrix based on Euler angle convention Head-Pitch-Roll.
This requires setting the rotation.order property of the object to "YXZ".

Keyboard controls:

• y - head +15 degrees
• Y - head -15 degrees
• x - pitch +15 degrees
• X - pitch -15 degrees
• z - roll +15 degrees
• Z - roll -15 degrees
• o - Reset to identity