LibWeb+WebContent: Forbid access to underlying type of CSSPixels

Although DistinctNumeric, which is supposed to abstract the underlying
type, was used to represent CSSPixels, we have a whole bunch of places
in the layout code that assume CSSPixels::value() returns a
floating-point type. This assumption makes it difficult to replace the
underlying type in CSSPixels with a non-floating type.

To make it easier to transition CSSPixels to fixed-point math, one step
we can take is to prevent access to the underlying type using value()
and instead use explicit conversions with the to_float(), to_double(),
and to_int() methods.

Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.cpp

@@ -214,7 +214,7 @@ Optional<float> SVGGraphicsElement::stroke_width() const
             }
         }
         auto scaled_viewport_size = (viewport_width + viewport_height) * 0.5;
-        return width->to_px(*layout_node(), scaled_viewport_size).value();
+        return width->to_px(*layout_node(), scaled_viewport_size).to_double();
     }
     return {};
 }