Expose EventLoop

[Source Code](https://github.com/rust-windowing/glutin/blob/bab33a84dfb094ff65c059400bed7993434638e2/glutin/src/platform_impl/android/mod.rs#L4)

As shown in the implementation above, it seems that the EventLoop should be exposed from d3d12-rs.

[Usage](https://github.com/imgui-rs/imgui-rs/blob/35342a0c8255a26d0f7aad99eb6498a796b92f32/imgui-examples/examples/support/mod.rs#L28)