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/Layout/SVGSVGBox.cpp

@@ -57,7 +57,7 @@ Optional<float> SVGSVGBox::calculate_intrinsic_aspect_ratio() const
 
         if (width != 0 && height != 0) {
             // 1. return width / height
-            return width.value() / height.value();
+            return width.to_double() / height.to_double();
         }
 
         return {};