Memory Leak - goroutine not cleaned up when stopping frame

Hi, first of all - really cool project! Thank you so much for sharing!

I've been trying to make animations with this package for educational purposes and have had success so far. I came across a memory leak in the package, specifically cleaning up the process goroutine defined in render/frame.go. I have no idea what the best way to fix this.

It seems that we're stuck on reading from the channel in the for loop in the process function.
This causes all the linked data to stay in memory when rendering multiple frames in a sequence.

I was able to fix the leak by modifying the Stop method in frame.go to close the channel off. I can't close it off right away because there might still be tracers which send to the channel, so I put a timer in. I realize this is bad, but I haven't yet looked at a better way to handle this.

```golang
//render/frame.go
func (f *Frame) Stop() {
if f.active.Set(false) {
for _, w := range f.workers {
w.stop()
}

go func() {
time.Sleep(time.Second * 10)
close(f.in)
}()
}
}
```