Add option for setting the PNG zlib compression level

Source/Core/Common/ImageC.c

@@ -0,0 +1,27 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "Common/ImageC.h"
+
+// Since libpng requires use of setjmp, and setjmp interacts poorly with destructors and other C++
+// features, this is in a separate C file.
+
+// The main purpose of this function is to allow specifying the compression level, which
+// png_image_write_to_memory does not allow.  row_pointers is not modified by libpng, but also isn't
+// const for some reason.
+bool SavePNG0(png_structp png_ptr, png_infop info_ptr, int png_format, png_uint_32 width,
+              png_uint_32 height, int level, png_voidp io_ptr, png_rw_ptr write_fn,
+              png_bytepp row_pointers)
+{
+  if (setjmp(png_jmpbuf(png_ptr)) != 0)
+    return false;
+
+  png_set_compression_level(png_ptr, level);
+  png_set_IHDR(png_ptr, info_ptr, width, height, 8, png_format, PNG_INTERLACE_NONE,
+               PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
+  png_set_rows(png_ptr, info_ptr, row_pointers);
+  png_set_write_fn(png_ptr, io_ptr, write_fn, NULL);
+  png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
+
+  return true;
+}