decodebin2: Fix group switching algorithm

decodebin2: Fix group switching algorithm

There were two issues with the previous decodebin2 group switching algorithm:

Issue 1: It operated with no memory of what has been drained or not, leading to
multiple checks for chains/groups that were already drained.

Issue 2: When receiving an EOS, it only detected that a higher-level chain
was drained if it contained the pad receiving the EOS.

The following modifications have been applied:
- a new drained property has been added to GstDecodeChain
- both drained properties of chain/group are set as soon as they are detected
- the algorithm now tests agains these values

See https://bugzilla.gnome.org/show_bug.cgi?id=685938