Rewrite cellPngDec to use libpng

Improvements made:
* Supports tEXt chunks
* Supports chunk information
* Supports 16-bit depth image packing (fixes #1127)
* Supports fixed alpha values
* And more.

.travis.yml

@@ -53,7 +53,7 @@ before_install:
     fi;
 
 before_script:
- - git submodule update --init asmjit ffmpeg rsx_program_decompiler GSL
+ - git submodule update --init asmjit ffmpeg rsx_program_decompiler GSL libpng
  - mkdir build
  - cd build
  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then cmake ..; else cmake .. -DLLVM_DIR=/usr/local/opt/llvm36/lib/llvm-3.6/share/llvm/cmake; fi