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

Reset camera._movingToActor to false when camera._mode is changed to
kNormalCameraMode in ScummEngine::startScene(). This avoids the
situation described in bug #6679 in which the camera would move towards
Indy's position from the previous room after switching to room 64 (Book
of Maps) in a specific variant of Indy3 DOS/EGA (v1.0, according to
devtools/scumm-md5.txt).