LibDraw: Start work on a GIF decoder (not yet functional!)

Here comes the first part of a GIF decoder. It decodes up to the point
of gathering all the LZW-compressed data. The next step is to implement
decompression, and then turn the decompressed data into a bitmap using
the color maps, etc.

Libraries/LibDraw/GIFLoader.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibDraw/GraphicsBitmap.h>
+#include <LibDraw/ImageDecoder.h>
+
+RefPtr<GraphicsBitmap> load_gif(const StringView& path);
+RefPtr<GraphicsBitmap> load_gif_from_memory(const u8*, size_t);
+
+struct GIFLoadingContext;
+
+class GIFImageDecoderPlugin final : public ImageDecoderPlugin {
+public:
+    virtual ~GIFImageDecoderPlugin() override;
+    GIFImageDecoderPlugin(const u8*, size_t);
+
+    virtual Size size() override;
+    virtual RefPtr<GraphicsBitmap> bitmap() override;
+
+private:
+    OwnPtr<GIFLoadingContext> m_context;
+};