decodebin3: fix collection refcounting

decodebin3: fix collection refcounting

My collection leak fix 83f30627cd9460157935e7e9603c60a15555967e
introduced a crash in this scenario: audiotestsrc ! decodebin3 ! fakesink

The reference handling of collection in decodebin3 wasn't very clear and
my attempt to fix the leak introduced a regression where we went one
reference short in some other scenarios.

Fixing this by:
- Giving a strong reference to DecodebinInput making things clearer
- Fixing get_merged_collection() which was sometimes returning an
  existing reference and sometimes a new one.

https://bugzilla.gnome.org/show_bug.cgi?id=769080