ImageDecoder+LibImageDecoder+LibWeb: Support animations in ImageDecoder

The ImageDecoder service now returns a list of image frames, each with
a duration value.

The code for in-process image decoding is removed from LibWeb, an all
image decode requests are sent out-of-process to ImageDecoder. :^)

This won't scale super well to very long and/or large animations, but
we can work on improving that separately. The main goal here is simply
to stop doing any image decoding inside LibWeb.

Fixes #5165.

Userland/Libraries/LibWeb/Loader/ImageLoader.h

@@ -38,7 +38,8 @@ public:
 
     void load(const URL&);
 
-    const Gfx::Bitmap* bitmap() const;
+    const Gfx::Bitmap* bitmap(size_t index) const;
+    size_t current_frame_index() const { return m_current_frame_index; }
 
     bool has_image() const;