Cache to speedup loading multiple instances of the same scene URL

[DP News-Robot]

New property allows to load the scene contents through a shared cache. If you have multiple loaded from the same URL, this is a sure way to speedup loading of them. Testcase: examples/viewport_and_scenes/occlusion_culling, where I set up multiple buildings and creatures from the same glTF. Alternative: You can actually achieve even more optimization (loading time, …