WebPAnimEncoder: Restore canvas before evaluating blending possibility.
WebPAnimEncoder: Restore canvas before evaluating blending possibility.

Without this, the canvas may have been modified in-between two
GenerateCandidate() calls.

Change-Id: I0364a269caabdf282c30a8fa3c61896f0247342e

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