Change the rule of picking UV mode in MBAnalyzeBestUVMode()
Change the rule of picking UV mode in MBAnalyzeBestUVMode()

Pick the mode with the smallest alpha.
It only affects m0, in which case the mode decision is not re-examined
later in VP8Decimate(). Tests on some natural content png images show
PSNR increase as well as visual quality improvement.

Change-Id: Iea997e718cd7477160fa05eb7cfb35f4cec2fa9a

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