Some PNG input contain chunks larger than libpng's default
memory-alloc limit (8M).
Raise this limit reasonably if it looks like the input bitstream
is larger than libpng's default limit.
BUG=webp:497
Change-Id: I2c9fbed727424042444b82cbf15e0781cefb38dc
(cherry picked from commit 8696147da438babdc30ea6fd291d1e8abb407828)