Softlock when calling stop after streams are finished.

[LeifAndersen]

The following code exhibits this behavior:

#lang racket

(require video/render
         video/base)

(define video
  (color "green"
          #:properties (hash "start" 10
                             "end" 20)))

(define render (make-object render% video))
(send render setup (make-render-settings))
(send render start-rendering)

(sleep 0.1)
(displayln "stopping")
(send render stop-rendering)
(displayln "here")

This appears to show two problems with video:

1. The renderer immediately returns eof, which is wrong.
2. When the stop happens after a brief pause (about 100 ms), it causes Video to soft lock.

[LeifAndersen]

To be clear, it gets stuck on this line:

(send render stop-rendering)

[LeifAndersen]

Commit da79dc2243359b4f39ea3b919d90d98f49f00d22 fixes this.