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/CSS/Screen.cpp

@@ -42,10 +42,10 @@ Gfx::IntRect Screen::screen_rect() const
 {
     auto screen_rect_in_css_pixels = window().page()->web_exposed_screen_area();
     return {
-        screen_rect_in_css_pixels.x().value(),
-        screen_rect_in_css_pixels.y().value(),
-        screen_rect_in_css_pixels.width().value(),
-        screen_rect_in_css_pixels.height().value()
+        screen_rect_in_css_pixels.x().to_int(),
+        screen_rect_in_css_pixels.y().to_int(),
+        screen_rect_in_css_pixels.width().to_int(),
+        screen_rect_in_css_pixels.height().to_int()
     };
 }