Common: Add SavePNG() function that writes PNGs using the simplified libpng API.

Source/Core/Common/Image.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <string>
 #include <vector>
 
 #include "Common/CommonTypes.h"
@@ -12,4 +13,13 @@ namespace Common
 {
 bool LoadPNG(const std::vector<u8>& input, std::vector<u8>* data_out, u32* width_out,
              u32* height_out);
-}
+
+enum class ImageByteFormat
+{
+  RGB,
+  RGBA,
+};
+
+bool SavePNG(const std::string& path, const u8* input, ImageByteFormat format, u32 width,
+             u32 height, int stride = 0);
+}  // namespace Common