LibWeb: Improve inline flow around floating boxes

This patch combines a number of techniques to make inline content flow
more correctly around floats:

- During inline layout, BFC now lets LineBuilder decide the Y coordinate
  when inserting a new float. LineBuilder has more information about the
  currently accumulated line, and can make better breaking decisions.

- When inserting a float on one side, and the top of the newly inserted
  float is below the bottommost float on the opposite side, we now reset
  the opposite side back to the start of that edge. This improves
  breaking behavior between opposite-side floats.

- After inserting a float during inline layout, we now recalculate the
  available space on the line, but don't adjust X offsets of already
  existing fragments. This is handled by update_last_line() anyway,
  so it was pointless busywork.

- When measuring whether a line can fit at a given Y coordinate, we now
  consider both the top and bottom Y values of the line. This fixes an
  issue where the bottom part of a line would bleed over other content
  (since we had only checked that the top Y coordinate of that line
  would fit.)

There are some pretty brain-dead algorithms in here that we need to make
smarter, but I didn't want to complicate this any further so I've left
FIXMEs about them instead.

Userland/Libraries/LibWeb/Layout/InlineFormattingContext.h

@@ -30,6 +30,7 @@ public:
     float leftmost_x_offset_at(float y) const;
     float available_space_for_line(float y) const;
     bool any_floats_intrude_at_y(float y) const;
+    bool can_fit_new_line_at_y(float y) const;
 
     float effective_containing_block_width() const { return m_effective_containing_block_width; }