Reused mach.Options for run time options. It is set with
Engine.setOptions function. ``pub const options`` on top level App has
no effect and will be ignored completely.
Added a blank struct StartupOptions which would be used for startup time
options in future. Currently they aren't used for anything.
GpuDriver
Core and GpuDriver both are merged into one type called Platform. Also
previously the fields and methods which were called as
``engine.core.field`` will now be ``engine.field`` i.e an extra layer is
removed.