RFC: Add tunneling support.

Submitted by deathsimple@vodafone.de

Link to original bug (#726325)

Description

Still in a quite early stage of development, but the basic idea should be clear now.

All omx components that support tunneling should use the omx buffer pool on their sinks in their propose_allocation function. The source components can then detect that the OMX buffer pool is in use and establish direct tunneling.

Please review and comment.