PNG decoder: handle gAMA chunk
PNG decoder: handle gAMA chunk

Apply gamma correction to the decoded RGB values.

This handles corner cases where the PNG file doesn't have
a standard 1/2.2 gamma value.

BUG=webp:369

Change-Id: I9907b6e2c458002de7c26d0b9e416278cca33990

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