AnimEncoder: Correctly skip a frame when sub-rectangle is empty.
AnimEncoder: Correctly skip a frame when sub-rectangle is empty.

Change-Id: I0d288bd9561b48cf5a1eae92a1b7106ba44c664e
(cherry picked from commit 1cc79e92ac74337aa4102a3128fa9451ef4b5fd0)

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