Negotiation
-----------

Negotiation happens when elements want to push buffers and need to decide
on the format. 

For any two linked pads there are different ways to agree on the format but
in general the source pad will propose a format and the sink format accepts
or rejects.

             src              sink
              |                 |
              |  accepts?       |
	      |---------------->|
              |      yes        |
              |<----------------|
              |                 |
 get buffer   |  alloc_buf      |
 from pool    |---------------->| 
 with type    |                 | Create buffer of type.
              |                 |
 check type   |<----------------|
 and use      |                 |
              |  push           |
 push buffer  |---------------->| Receive type, reconfigure to
 with new type|                 | process type.
              |                 |

Use case:


videotestsrc ! xvimagesink

  1) Who decides what format to use?
   - src pad always decides, by convention. sinkpad can suggest a format
     by putting it high in the getcaps function GstCaps. 
   - since the src decides, it can always choose something that it can do,
     so this step can only fail if the sinkpad stated it could accept
     something while it later on couldn't.

  2) When does negotiation happen?
   - before srcpad does a push, it figures out a type as stated in 1), then 
     it calls the pad alloc function with the type. The sinkpad has to 
     create a buffer of that type, src fills the buffer and sends it to sink.
   - since the sink stated in 1) it could accept the type, it will be able to
     create a buffer of the type and handle it.
   - sink checks media type of buffer and configures itself for this type.
     
  3) How can sink request another format?
   - sink asks if new format is possible for the source.
   - sink returns buffer with new type in allocfunction.
   - src receives buffer with new type, reconfigures and pushes.
   - sink can always select something it can create and handle since it takes
     the initiative. src should be able to handle the new type since it said
     it could accept it.

videotestsrc ! queue ! xvimagesink

  - queue implements an allocfunction, proxying all calls to its srcpad peer.
  - queue proxies all accept and getcaps to the other peer pad.