LibWeb: Don't draw text fragments that would be clipped by the painter

This avoids a ton of work when painting large documents. Even though it
would eventually get clipped by the painter anyway, by bailing out
earlier, we avoid a lot more work (UTF-8 iteration, OpenType lookups,
etc).

It would be even nicer if we could skip entire line boxes, but we don't
have a fast way to get the bounding rect of a line box at the moment.

Userland/Libraries/LibWeb/Painting/PaintContext.h

@@ -35,6 +35,8 @@ public:
     void set_device_viewport_rect(DevicePixelRect const& rect) { m_device_viewport_rect = rect; }
     CSSPixelRect css_viewport_rect() const;
 
+    [[nodiscard]] bool would_be_fully_clipped_by_painter(DevicePixelRect) const;
+
     bool has_focus() const { return m_focus; }
     void set_has_focus(bool focus) { m_focus = focus; }