Sync-patch with libwebp ver 0.4.2

Sync-patch with libwebp ver 0.4.2 (change#I32a22786f)
Also cherry-picked one-line fix for Alpha-decode bug (ref: 'src < src_end')

Verfied the build on volantis/flounder (N9), razor/flo (N7), hammerhead (N5) and nakasi
(N7-old) and verified that the corrupted image rendered correctly on
Razor and Volantis devices.

Change-Id: I1d013d5b8d2236957c85c2cd2cdfa083733b5215
(cherry picked from commit 8c098653157979e397d3954fc2ea0ee43bae6ab2)
33 files changed