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.

CMakeLists.txt

@@ -12,6 +12,7 @@ if(NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
 endif()
 
 add_subdirectory( asmjit )
+add_subdirectory( libpng )
 # TODO: do real installation, including copying directory structure
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE "${PROJECT_BINARY_DIR}/bin")
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG "${PROJECT_BINARY_DIR}/bin")