Case 1:


---------------------------------------------------
| pipeline                                        |
| ---------------                ---------------- |
| | bin         |                | thread       | |
| | ----------- |  ------------  | ------------ | |
| | | fakesrc | |  | queue    |  | | fakesink | | |
| | |     src>|-|--|<sink src>|--|-|<sink     | | |
| | ----------- |  ------------  | ------------ | |
| ---------------                ---------------- |
---------------------------------------------------

Pipeline manages: fakesrc, queue
Thread manages: fakesink
Both forced to use cothreads.

First thing the thread does is try to pull from the queue.  Because it's a chain function, it runs in
_chain_wrapper, which calls gst_pad_pull().