avviddec: improve buffer handling and semantics

... so as to focus on providing *a* buffer rather than one (too) tied
to a frame, in particular allowing multiple allocations related to a frame.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=697806
Some compiler warning fixes to satisfy XCode compiler

https://bugzilla.gnome.org/show_bug.cgi?id=720513
Automatic update of common submodule

From dbedaa0 to d48bed3
Merge remote-tracking branch 'upstream/master' into ruih/master
Merge pull request #5 from cablelabs/merge-2013-12-23

Merge 2013 12 23