Update to Babylon.js 7.6.2

[bghgary]

Updating to Babylon.js version 7.0.0 reveals a whole slew of issues when running the validation tests because #14868 broke the render loop stopping behavior when running in native. Native does not implement a way to cancel a requested animation frame. This missing canceling functionality should be added to native someday, but, in the meantime, #15086 will make it behave like it did before.

These changes fix potential issues in the code regardless of what happened on the JS side.

• Update to latest JsRuntimeHost with lots of fixes.
• Call SetRenderResetCallback with null to clear the callback that can cause a crash on shutdown.
• Update a few validation tests to not use render count.
• Update scripts to use const/let instead of var.
• Update validation script to handle some errors better and clean up dead code.
• Add unhandled exception handler to Playground app so that validation test will report correct errors and exit correctly.
• Change core graphics device implemenation to update bgfx state right before requesting screenshots. This will ensure the screenshot is the right size if the resolution has changed.
• Fix TestUtils:WritePNG to return an error when the byte length is an unexpected size instead of silently failing.
• Change TestUtils::SetTitle to update the HWND title on a background thread to avoid dead lock with the main thread.
• Update dynamic texture clip test to newest version that will fail if the feature is not working.