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

remove unnecessary conversion

src/core/frontend/image_interface.h

@@ -0,0 +1,28 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <string>
+#include <vector>
+#include "common/common_types.h"
+#include "common/logging/log.h"
+
+namespace Frontend {
+
+class ImageInterface {
+public:
+    // Error logging should be handled by the frontend
+    virtual bool DecodePNG(std::vector<u8>& dst, u32& width, u32& height, const std::string& path) {
+        LOG_CRITICAL(Frontend, "Attempted to decode PNG without an image interface!");
+        return false;
+    };
+    virtual bool EncodePNG(const std::string& path, const std::vector<u8>& src, u32 width,
+                           u32 height) {
+        LOG_CRITICAL(Frontend, "Attempted to encode PNG without an image interface!");
+        return false;
+    };
+};
+
+} // namespace Frontend