decoder: reset segmentation map on keyframes

Refactoring some of the mode decoding logic introduced a bug where
the segmentation maps would not be properly reset on keyframes.

http://code.google.com/p/webm/issues/detail?id=378

The text of the bug is somewhat misleading as I initially read it to
imply the bug was present in v0.9.7-p1 (Cayuga), but note the text
"master", which indicates this was something subsequent. This issue
bisects back to v0.9.7-p1-84-ga99c20c, so unfortunately it was broken
during the Duclair release.

Thanks to Alexei Leonenko for investigating the root cause.

Change-Id: I9713c9f070eb37b31b3b029d9ef96be9b6ea2def