LibWeb: Fix the x coordinate of a block after a float

The margin from the containing blocks shouldn't be included in the
amount by which we increment x after a float was places. That coordinate
should be relative to the containing block.

Fixes the comments layout on https://lobste.rs.

Userland/Libraries/LibWeb/Layout/FormattingContext.h

@@ -107,6 +107,16 @@ protected:
         CSSPixels right { 0 };
     };
 
+    struct SpaceUsedAndContainingMarginForFloats {
+        // Width for left / right floats, including their own margins.
+        CSSPixels left_used_space;
+        CSSPixels right_used_space;
+        // Left / right total margins from the outermost containing block to the floating element.
+        // Each block in the containing chain adds its own margin and we store the total here.
+        CSSPixels left_total_containing_margin;
+        CSSPixels right_total_containing_margin;
+    };
+
     struct ShrinkToFitResult {
         CSSPixels preferred_width { 0 };
         CSSPixels preferred_minimum_width { 0 };