add image interface, remove lodepng from video_core/core, address more comments, fix comments

remove unnecessary conversion

src/citra_qt/qt_image_interface.h

@@ -0,0 +1,14 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "core/frontend/image_interface.h"
+
+class QtImageInterface final : public Frontend::ImageInterface {
+public:
+    bool DecodePNG(std::vector<u8>& dst, u32& width, u32& height, const std::string& path) override;
+    bool EncodePNG(const std::string& path, const std::vector<u8>& src, u32 width,
+                   u32 height) override;
+};