add option --no_instanced_collision_shapes, this only applies to the 2 benchmark screnes at the moment
always use the user specified b3Config file, so the settings can be set at runtime, not just at compile time
adjust default constants in b3Config (needs more tweaking, ideally at run-time)
apply forces
copy linear/angular velocity every frame at the CPU side, initial velocity works now
hook up setGravity
Note: the 'stepSimulation' for GPU only simulates a single simulation frame.
