backward_references: quiet double->int warning
backward_references: quiet double->int warning

since:
059aab4 Fix a compression regression for images with long uniform
regions.

Change-Id: I1783a74220961e8bc3bb42696e3412fe4bfc4ddb

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp