basemixer: Connect the mixer src pads to the mixerendpoint sink pads

Change-Id: If6a9e948f0d6a4e8a2129d122d347d314a078eeb
basemixer: deactivates the pad before remove it

Change-Id: I6e737af4dee25e4228fd56d071ecb9cc7b92f53a
mainmixer: Add test for mainmixer element

Change-Id: I0e98542a6614b8dcfe1106ab870089d0c13dd2a4
kmscompositemixer: add skelleton class

Change-Id: Ibeed43ad5653264a8fb553478d7cf4b4eef281d4