Refactor APNG loader to use libpng-based implementation

Replaces the custom APNG loading and saving logic in apngloader.cpp with a new implementation based on libpng, moved to apng_png.hpp. Updates CMakeLists.txt to require PNG, and ensures libpng is built with APNG support. This change modernizes and simplifies APNG handling, improving maintainability and leveraging standard library support.

Author: dudantas

src/CMakeLists.txt

@@ -180,6 +180,7 @@ add_definitions(-D"BUILD_REVISION=\\\"${BUILD_REVISION}\\\"")
 find_package(OpenSSL QUIET)
 find_package(PhysFS REQUIRED)
 find_package(ZLIB REQUIRED)
+find_package(PNG REQUIRED)
 find_package(phmap REQUIRED)
 find_package(absl CONFIG REQUIRED)
 find_package(Protobuf REQUIRED)
@@ -509,6 +510,7 @@ if(MSVC)
           ${CMAKE_THREAD_LIBS_INIT}
           ${PHYSFS_LIBRARY}
           ${ZLIB_LIBRARY}
+          PNG::PNG
           ${NLOHMANN_JSON_LIBRARY}
           ${OPENGL_LIBRARIES}
           ${DirectX_LIBRARY}
@@ -555,6 +557,7 @@ elseif(ANDROID)
           ${LUAJIT_LIBRARY}
           ${PHYSFS_LIBRARY}
           ${ZLIB_LIBRARY}
+          PNG::PNG
           ${NLOHMANN_JSON_LIBRARY}
           ${EGL_LIBRARY}
           ${OGG_LIBRARY}
@@ -602,6 +605,7 @@ elseif(WASM)
           ${LUA_LIBRARY}
           ${PHYSFS_LIBRARY}
           ${ZLIB_LIBRARY}
+          PNG::PNG
           ${PROTOBUF_LIBRARY}
           ${NLOHMANN_JSON_LIBRARY}
           ${GLEW_LIBRARY}