LibGfx: Prevent calling to_type<T>() on Line/Point/Rect/Size<T>

Also, add `Line::to_type<T>()` since that was missing.

Calling to_type() with the same type as the existing object accomplishes
nothing except wasting some cycles and making the code more verbose,
and it is hard to spot. Nobody does this in the code currently
(yay!) but I made this mistake repeatedly when doing my step-by-step
CSS Pixels conversion, so let's make it easier to catch them.

Userland/Libraries/LibGfx/Rect.h

@@ -949,7 +949,8 @@ public:
     }
 
     template<typename U>
-    [[nodiscard]] ALWAYS_INLINE Rect<U> to_type() const
+    requires(!IsSame<T, U>)
+        [[nodiscard]] ALWAYS_INLINE Rect<U> to_type() const
     {
         return Rect<U>(*this);
     }