with loop_compatibility disabled (the default), non-zero loop counts
will be incremented by 1 for browser rendering compatibility. the max,
65535, is a special case as the muxer will fail if it is exceeded; avoid
increasing the limit in this case. this isn't 100% correct, but should
be close enough given the high number of iterations.
BUG=webp:382
Change-Id: Icde3e98a58e9ee89604a72fafda30ab71060dec5