# Objective
- Fixes#19910.
## Solution
- First, allow extraction function to be FnMut instead of Fn. FnMut is a
superset of Fn anyway, and we only ever call this function once at a
time (we would never call this in parallel for different pairs of worlds
or something).
- Run the `RenderStartup` schedule in the extract function with a flag
to only do it once.
- Remove all the `MainRender` stuff.
One sad part here is that now the `RenderStartup` blocks extraction. So
for pipelined rendering, our simulation will be blocked on the first
frame while we set up all the rendering resources. I don't see this as a
big loss though since A) that is fundamentally what we want here -
extraction **has to** run after `RenderStartup`, and the only way to do
better is to somehow run `RenderStartup` in parallel with the first
simulation frame, and B) without `RenderStartup` the **entire** app was
blocked on initializing render resources during Plugin construction - so
we're not really losing anything here.
## Testing
- I ran the `custom_post_processing` example (which was ported to use
`RenderStartup` in #19886) and it still works.
2c6cf9a597