LibWeb+Browser: Decode non-animated images out-of-process :^)

We now use the ImageDecoder service in LibWeb for everything except
GIF images (we'll have to deal with them later, ofc.)

This has a little bit of overhead but we should be able to optimize
it until it becomes negligible.

Libraries/LibWeb/DOM/HTMLImageElement.h

@@ -49,7 +49,6 @@ public:
     String src() const { return attribute(HTML::AttributeNames::src); }
 
     const Gfx::Bitmap* bitmap() const;
-    const Gfx::ImageDecoder* image_decoder() const;
 
 private:
     void animate();