LibWeb: Resolve double-position linear-gradient() color stops

These just resolve to an extra color stop.
Something like "red 10% 40%"  is just shorthand for "red 10%, red 40%".

Userland/Libraries/LibWeb/Painting/GradientPainting.h

@@ -16,7 +16,7 @@ namespace Web::Painting {
 
 struct ColorStop {
     Gfx::Color color;
-    float position = 0;
+    float position = AK::NaN<float>;
     Optional<float> transition_hint = {};
 };