WebPAnimEncoder: Fix for single-frame optimization.
WebPAnimEncoder: Fix for single-frame optimization.

Change-Id: I2da8dc34ab9589b58cde0ecb05d71357d77f6b25

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