Add patch for PNG needed on mac

deps/PNG/PNG.cmake

@@ -5,12 +5,18 @@ else ()
     set(_disable_neon_extension "")
 endif ()
 
+set(_patch_step "")
+if (APPLE)
+    set(_patch_step PATCH_COMMAND ${PATCH_CMD} ${CMAKE_CURRENT_LIST_DIR}/PNG.patch)
+endif ()
+
 prusaslicer_add_cmake_project(PNG 
     # GIT_REPOSITORY https://github.com/glennrp/libpng.git 
     # GIT_TAG v1.6.35
     URL https://github.com/glennrp/libpng/archive/refs/tags/v1.6.35.zip
     URL_HASH SHA256=3d22d46c566b1761a0e15ea397589b3a5f36ac09b7c785382e6470156c04247f
     DEPENDS ${ZLIB_PKG}
+    "${_patch_step}"
     CMAKE_ARGS
         -DPNG_SHARED=OFF
         -DPNG_STATIC=ON

deps/PNG/PNG.patch

@@ -0,0 +1,26 @@
+Common subdirectories: ../libpng-1.6.35-orig/arm and ./arm
+Common subdirectories: ../libpng-1.6.35-orig/contrib and ./contrib
+Common subdirectories: ../libpng-1.6.35-orig/intel and ./intel
+Common subdirectories: ../libpng-1.6.35-orig/mips and ./mips
+Only in ./: PNG.patch
+diff -u ../libpng-1.6.35-orig/pngrutil.c ./pngrutil.c
+--- ../libpng-1.6.35-orig/pngrutil.c	2018-07-15 20:58:00.000000000 +0200
++++ ./pngrutil.c	2021-03-24 15:59:38.687108444 +0100
+@@ -422,13 +422,6 @@
+             png_ptr->flags |= PNG_FLAG_ZSTREAM_INITIALIZED;
+       }
+ 
+-#if ZLIB_VERNUM >= 0x1290 && \
+-   defined(PNG_SET_OPTION_SUPPORTED) && defined(PNG_IGNORE_ADLER32)
+-      if (((png_ptr->options >> PNG_IGNORE_ADLER32) & 3) == PNG_OPTION_ON)
+-         /* Turn off validation of the ADLER32 checksum in IDAT chunks */
+-         ret = inflateValidate(&png_ptr->zstream, 0);
+-#endif
+-
+       if (ret == Z_OK)
+          png_ptr->zowner = owner;
+ 
+Common subdirectories: ../libpng-1.6.35-orig/powerpc and ./powerpc
+Common subdirectories: ../libpng-1.6.35-orig/projects and ./projects
+Common subdirectories: ../libpng-1.6.35-orig/scripts and ./scripts
+Common subdirectories: ../libpng-1.6.35-orig/tests and ./tests