pngdec: raise memory limit if needed

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
1 file changed