Replace Napi methods with properties where possible

Methods like `NativeEngine::GetRenderWidth` start to appear in CDT profiling. It's called (at least) once per frame even with that few calls, it's slow enough to rise in the top methods.
(https://github.com/BabylonJS/BabylonNative/blob/4fd6df8541c5bc0ae98a9db7879917f46dbc0eb2/Plugins/NativeEngine/Source/NativeEngine.cpp#L1484)

Replace with properties anywhere possible. Only update the napi value when it changes.