vp8: fix loop filter level clamping

the loop filter level is transmitted as 6-bits + sign so needs to be clamped in
the delta + absolute case.

BUG=https://bugzilla.mozilla.org/show_bug.cgi?id=1224363

Change-Id: Icbdca4fdbf043466429bd5c9d59dbe913bf153bc
1 file changed