Make render target more consumable.

[John]

Mostly we use the RenderTarget stuff for probes and it's use is very internal to the engine, however I can see the use case for needing to expose it more. A possible API for that might be:

let render_target = RenderTarget:new(.....)
pipeline_manager.set_target(render_target);
app.render_now();
pipeline_manager.reset_target();

Or in the case of custom rendering:

let mut render_schedule =
    Schedule::builder()
        .add_system(crate::systems::MySystem::create())
        .flush()
        .add_thread_local_fn(harmony::graphics::systems::render::create())
        .build();

let render_target = RenderTarget:new(....)
pipeline_manager.set_target(render_target);
render_schedule.execute(...);
pipeline_manager.reset_target();