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/CSS/GridTrackSize.cpp

@@ -14,7 +14,7 @@ GridSize::GridSize(LengthPercentage length_percentage)
     : m_type(Type::LengthPercentage)
     , m_length_percentage(length_percentage) {};
 
-GridSize::GridSize(float flex_factor)
+GridSize::GridSize(double flex_factor)
     : m_type(Type::FlexibleLength)
     , m_length_percentage { Length::make_px(0) }
     , m_flex_factor(flex_factor)