misc:
fix units not moving in FPS mode
fix units moving in circles near obstacles (#3718, weird)
fix aircraft jittering at high altitude
add vector constants for the "right" (x-axis) and "forward" (z-axis) directions
SolidObject::Move1D --> /dev/null
SolidObject::Move3D --> SolidObject::Move
Merge branch 'develop' of github.com:spring/spring into develop