Add option for setting the PNG zlib compression level

Source/Core/Common/ImageC.h

@@ -0,0 +1,15 @@
+// Copyright 2021 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <png.h>
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C"
+#endif
+    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);