add a toggle to switch to FPS-style view controls in the modelviewer
build the required rotation matrix directly instead of building & inverting
change modelviewer mouselook from 'FPS style' to 'flight style' controls

FPS style: rotations are around global 'Y' (turning) and local 'X' (looking
up or down). Flight style: rotations are around local 'Y' (yaw) and local
'X' (pitch).
modelviewer: in mouselook mode, hold right mouse button to roll
modelviewer: rename fpsViewControls to mouselookEnabled

It doesn't follow normal FPS view conventions any more.
Merge remote-tracking branch 'johnbartholomew/modelviewer-fps-view'