The offset *can* be negative, but the sanitizer reports strange
address behaviour when row_offset is unsigned size_t.
For safety, use int64_t instead (probably overkill. int32_t is probably ok).
Change-Id: I1bd424bfdb5447b3839f40679581d6bdea075320
this avoids duplicates between these trees and dsp/, e.g., enc/tree.c,
dec/tree.c, making pulling the whole library source tree into one target
possible
BUG=webp:279
Change-Id: I060a614833c7c24ddd37bf641702ae6a5eef1775