Feat: implement a "small-steps" velocity-based constraints solver + joint improvements

[sebcrozet]

This is a complete rework of Rapier’s constraints solver. This has been cooking for quite a while now and I think it is time to be published, even though the parallel version has not been fully done yet (it will work, but with only island-based parallelism).

The idea is to combine the substep approach to solving constraints, similar to the latest known improvements of XPBD, but applied to Rapier’s velocity-based constraints formulation. In other words, it does not use the PBD formalism, but will solve Rapier’s pre-existing velocity-based constraints by subdividing the timestep into n substeps with 1 force calculation iteration per substep, instead of running n force calculation iteration for the full timestep. This result in a significantly improved convergence of joints, especially in case of large mass ratios due to the more non-linear handling of constraints.

Other changes include:

• Joint angular motors and limits have been fixed to actually use the user-given angle (instead of its half value).
• Torque generated by angular motors with a target position are now linear with the angle error (before, it would gradually slown down as the motor reaches its target position).
• It is now possible to increase the number of solver iteration per-rigid-body (RigidBodyBuilder::additional_solver_iterations, RigidBody::set_additional_solver_iterations). This allows increasing simulation accuracy on a subset of objects instead of having increase the global solver iteration count, increasing computational cost on unrelated objects.
• SpringJoint has been added. More generally, motors on joints with coupled linear axes are now supported.

The following video shows the stability improvements on a high-mass-ratio pendulum scenario. The large ball is 1000x heavier than the small ones:

• First video: with 20 iterations of the new solver, the simulation is stable.
• Second video: even with 4000 iteration of the old solver, the simulation breaks down immediately.

chain-large-mass-ratio-new-solver-20-iterations.mp4

chain-large-mass-ratio-old-solver-4000-iterations.mp4

Here is a video of the new spring joint. The springs, with various damping coefficients, are attached to the balls. The cubes are just object falling freely on top of the balls:

spring-joints.mp4

Fix #241