LibWeb: Make CSSPixels and Length use 64-bit (double) floating point

This fixes a plethora of rounding problems on many websites.
In the future, we may want to replace this with fixed-point arithmetic
(bug #18566) for performance (and consistency with other engines),
but in the meantime this makes the web look a bit better. :^)

There's a lot more things that could be converted to doubles, which
would reduce the amount of casting necessary in this patch.
We can do that incrementally, however.

Userland/Libraries/LibWeb/HTML/SourceSet.h

@@ -15,7 +15,7 @@ namespace Web::HTML {
 // https://html.spec.whatwg.org/multipage/images.html#image-source
 struct ImageSource {
     struct PixelDensityDescriptorValue {
-        float value { 0 };
+        double value { 0 };
     };
 
     struct WidthDescriptorValue {
@@ -28,7 +28,7 @@ struct ImageSource {
 
 struct ImageSourceAndPixelDensity {
     ImageSource source;
-    float pixel_density { 1.0f };
+    double pixel_density { 1.0f };
 };
 
 // https://html.spec.whatwg.org/multipage/images.html#source-set