SCUMM: Fix bug #6679 - INDY3 DOS-EGA: book of maps graphic glitch

This resets state variable camera._movingToActor to false when
camera._mode is changed to kNormalCameraMode inside
ScummEngine::startScene(). That makes sure the camera won't follow an
actor's last known position after the switch to certain rooms, which was
the cause of the strange behaviour described in bug #6679 for a specific
variant of Indy3 DOS/EGA (v1.0, according to devtools/scumm-md5.txt).

For a more detailed explanation of the issue, please have a look at bug #6679.