Some constraint issues

[Kanttori]

Hi!

First off, thanks to everyone concerned for the great lib.

Now, I'm working on some of the missing rigidbody constraints in blender2.49 (I'm waiting for 2.5's pyapi to be somewhat more stable:) and came across the following small issues with bullet 2.77 on blender game-engine and my own code which loads the bullet exports:

- btGeneric6DofConstraint is unstable/"wrong" when using D6_USE_FRAME_OFFSET = true with this setup

6dof_issue.bullet

(without D6_USE_FRAME_OFFSET it looks to be correct).

- btHingeConstraint moves a rigidbody to infinity when angularOnly is true and the rigidbody is attached to static object.

- In btConeTwistConstraint swing-axis is not constrained when the pivot point is on the same plane defined by said swing-axis and the center of mass, if the other swing-axis value is less than the fixed threshold (radian 0.05 or in degrees ~2.86). The rigidbody should just swing along the first axis until it's constrained and always "rest" on the second axis since it's in balance (and should be constrained to be fixed anyway), but atm. it freely rotates around the first axis incorrect

tweed.bullet

vs correct

tweed2.bullet

.