This problem is caused by discontinuities in the conversion
orientation matrix -> euler angles: the angle sign can
switch and thus the direction of the rotation produced
by the dRot Ipo.
To avoid this bug, the matrix->euler conversion must be
avoided during the game. I took the following approach that
is compatible with Blender (identical effect in the game and
in the 3D view):
- no change in Add mode: Rot and dRot are treated as additional
rotation to the orientation at the start of the Ipo. There is
no matrix->euler conversion and thus no discontinuities.
- Rot Ipo are treated as absolute rotation. All 3 axis should
be specified but if they are not, the startup object orientation
will be used to set the unspecified axis. By doing a matrix->
euler conversion once at the start, the discontinuities are
avoided. If there are also dRot curves, they are treated as
delta of the corresponding Rot curve or startup angle.
- dRot Ipo are treated as Add mode in Local axis.
Note about Add mode: Rot and dRot curves are treated identically
during the game. However, only dRot curves make sense because
they don't interfere with the object orientation in the 3D view.
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