This PR adds PhysX examples that are equivalent to all existing examples in aframe-physics system

[diarmidmackenzie]

Most of these examples could be implemented without core code changes.

There are just a couple of bug fixes

• rework of perf optimization, which turned out to have broken constraints
• fix a typo siffness-> stiffness, though even with this fix, springs still don't work :-(

Most examples work fine. The only exceptions are:

• we have issues with scale. Exhibited both in the sandbox example, and in the fact that I had to remove all scaling from the constraints example.
• springs example doesn't work. Soft linear constraints just don't seem to work correctly, and I don'y yet know why.

Those two problems might not be fast to solve, so I propose merging this PR as is, and raising separate issues to track those problems.

[diarmidmackenzie]

While this PR is under review, I've published the source branch at GitHub Pages here: https://diarmidmackenzie.github.io/physx/examples/

But note that links don't work - you'll need these instead:

https://diarmidmackenzie.github.io/physx/examples/basic/index.html https://diarmidmackenzie.github.io/physx/examples/sandbox/index.html https://diarmidmackenzie.github.io/physx/examples/compound/index.html https://diarmidmackenzie.github.io/physx/examples/constraints/index.html https://diarmidmackenzie.github.io/physx/examples/materials/index.html https://diarmidmackenzie.github.io/physx/examples/spring/index.html https://diarmidmackenzie.github.io/physx/examples/stress/index.html https://diarmidmackenzie.github.io/physx/examples/ttl/index.html https://diarmidmackenzie.github.io/physx/examples/pinboard/physx.html https://diarmidmackenzie.github.io/physx/examples/pinboard/ammo-vs-physx.html

[kylebakerio]

exciting! fantastic work, will try to look into this soon!