Native Bullet breaks TestBetterCharacter

[stephengold]

TestBetterCharacter works fine with JBullet, but with recent versions of Native Bullet, the character goes flying off into space. The root cause seems to be that the semantics of physicsCharacter.setGravity() are different. In JBullet, gravity is positive in the downward direction. In Native Bullet, it is positive in the upward direction.

[stephengold]

On closer inspection, it's not about different semantics. The issue is a bug in the test, which invokes physicsCharacter.setGravity() during the update phase. Moving setGravity() to the prePhysicsTick() phase resolves the issue.

[stephengold]

Fixed in 'master' branch.