Get reset (e.g. bgfx::shutdown) working with fast refresh

[ryantrem]

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As mentioned here: https://github.com/BabylonJS/BabylonNative/issues/523#issuecomment-736574732

Fast refresh is not working with the bgfx::shutdown call that happens while tearing down an engine in useEngine. As a quick fix, I am disabling the bgfx::shutdown call for dev mode builds (e.g. only release graphics resources in release builds). This isn't great, but it's proving rather difficult to fix due to details of how fast refresh works. We should try to fix this so both debug and release builds behave the same.