kmscompositemixer: Add a capsfilter in the inputs
kmscompositemixer: Add a capsfilter in the inputs

This capsfilter sets the input stream size to the output size.
All the streams size changes will be perform inside
the component. In this way, we avoid size negotiation errors
with previous agnostics.

Change-Id: I76aaef3631a4809abe4a3679337a5c8efec540b5

TribeMedia/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements