How can I only read N frames from filesrc?

[^_^Sisyphus^_^]

`num-buffers` has no effect on filesrc, the following will produce all
frames instead of just first 10.

```
gst-launch-1.0 -e filesrc location=/tmp/test.ts num-buffers=10 ! decodebin
! videoconvert ! pngenc ! multifilesink location='frame%05d.png'
```
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