sync version numbers to 0.4.2 release

libwebp{,decoder} - 0.4.2
libwebp libtool - 5.2.0
libwebpdecoder libtool - 1.2.0

mux/demux - 0.2.2
libtool - 1.2.0

(cherry picked from commit eec5f5f12179a4d950fc27aef1c70f087beade59)
(cherry picked from commit 857578a811ea96abfa3235e4e47dd1ab4726e655)

Change-Id: Ie9d10c68e28083674a8865ad8447b1a70dcea95d
Merge "sync version numbers to 0.4.2 release"