decodebin2: Handle flushing with multiple decode groups

decodebin2: Handle flushing with multiple decode groups

When an upstream element wants to flush downstream, we need to take
all chains/groups into consideration.

To that effect, when a FLUSH_START event is seen, after having it
sent downstream we mark all those chains/groups as "drained" (as if
they had seen a EOS event on the endpads).

When a FLUSH_STOP event is received, we check if we need to switch groups.
This is done by checking if there are next groups. If so, we will switch
over to the latest next_group. The actual switch will be done when
that group is blocked.

https://bugzilla.gnome.org/show_bug.cgi?id=606382