Tee support

Set of patches to help with some issues that arise in a tee'd scenario, that is, e.g. v4l2src whose output is then split by tee.  Problems encountered:
* a gst_buffer_make_writable in one branch (to alter metadata, e.g. videorate) triggers a memcpy into non-physical mapped memory
* if a (re-)negotiation is triggered (by event), e.g. if one branch is controlled dynamically, then streaming in v4l2src (streaming) fails due to buffer pool re-setup

Note that decoder allocated memory might then also be allowed shared (not in these patches), to support a subsequent tee.