smartYUV: fix and simplify the over-zealous stop criterion
smartYUV: fix and simplify the over-zealous stop criterion

We usually need at least 2 iterations to converge
(and usually not much more after that). Only 1 was not enough.

Change-Id: Iaf802ea81afa2596f4ba045c92f5eaff61623b7b

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