LibWeb: Paint 1x1 backgrounds as color fill instead of tiling bitmap

This yields a huge speedup on pages that use this weird but
not-entirely-uncommon technique.

Userland/Libraries/LibWeb/CSS/StyleValues/AbstractImageStyleValue.h

@@ -37,6 +37,8 @@ public:
 
     virtual bool is_paintable() const = 0;
     virtual void paint(PaintContext& context, DevicePixelRect const& dest_rect, ImageRendering) const = 0;
+
+    virtual Optional<Gfx::Color> color_if_single_pixel_bitmap() const { return {}; }
 };
 
 // And now, some gradient related things. Maybe these should live somewhere else.