Better playground scene, minor bug fixes

[qhdwight]

• Add real playground scene that I modeled in Blender and exported with GLTF.
• Better logic for not falling off ledge. Now you can move in direction along ledge as we only subtract off component that is overhanging. Still not happy as there are some edges cases where you can fall off.
• Internal calculations modify linvel in-place instead of keeping a separate variable. An example is when you run into a wall you want your velocity to diminish instead of remaining a pegged value. Then you don't glitch and fly off the wall when finally free.
• Beta step offset feature, disabled by default