Partially address babylonjs leak

[camillobruni]

We don't dispose of the babylon scene and engine which means we leak almost 100MB of cached data per iteration.

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[camillobruni]

For repro cases, setting iterationCount to 100 causes an OOM on chrome, given that we mostly leak ArrayBuffers, we are leaking a significant amount of memory.

workload	V8 14.6.80	JSC	Spidermonkey C147.0
babylonjs-scene-es6 before	73.5 ± 0.7(1.0%) pts	51.9 ± 1.3(2.45%) pts	72.9 ± 1.1(1.47%) pts
babylonjs-scene-es6 after	72.6 ± 1.3(1.78%) pts	50.67 ± 0.43(0.84%) pts	72.4 ± 1.2(1.63%) pts
babylonjs-startup-es6 before	30.73 ± 0.54(1.76%) pts	35.24 ± 0.45(1.28%) pts	29.93 ± 0.26(0.86%) pts
babylonjs-startup-es6 after	30.75 ± 0.45(1.47%) pts	35.04 ± 0.53(1.51%) pts	29.89 ± 0.38(1.28%) pts

V8 and mostly JSC do see to regress on babylonjs-scene-es6 after with the fix – I think everywhere else it's a wash (at least on my M1)

@kmiller68 could you make try to verify this on jsc on another machine?