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

remove unnecessary conversion

src/citra/generic_image_interface.cpp

@@ -0,0 +1,29 @@
+// Copyright 2019 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <lodepng.h>
+#include "common/logging/log.h"
+#include "generic_image_interface.h"
+
+bool GenericImageInterface::DecodePNG(std::vector<u8>& dst, u32& width, u32& height,
+                                      const std::string& path) {
+    u32 lodepng_ret = lodepng::decode(dst, width, height, path);
+    if (lodepng_ret) {
+        LOG_CRITICAL(Frontend, "Failed to decode {} because {}", path,
+                     lodepng_error_text(lodepng_ret));
+        return false;
+    }
+    return true;
+}
+
+bool GenericImageInterface::EncodePNG(const std::string& path, const std::vector<u8>& src,
+                                      u32 width, u32 height) {
+    u32 lodepng_ret = lodepng::encode(path, src, width, height);
+    if (lodepng_ret) {
+        LOG_CRITICAL(Frontend, "Failed to encode {} because {}", path,
+                     lodepng_error_text(lodepng_ret));
+        return false;
+    }
+    return true;
+}