LibWeb: Improve span column width distribution

Compute the contributions to a spanning cell width from each cell in the
span. This better handles uneven column widths, since each cell
contribution is proportional with its own width as opposed to the own
width of the first cell in the span.

This better matches the behavior of other browsers and further aligns
with the specification.

Tests/LibWeb/Layout/input/table/colspan-weighted-width-distribution.html

@@ -0,0 +1,18 @@
+<style>
+    table,
+    td {
+        border: 1px solid black;
+        border-spacing: 0px;
+        text-align: center;
+    }
+</style>
+
+<table>
+    <tr>
+        <td style="width: 180px;">A</td>
+        <td style="width: 20px;">B</td>
+    </tr>
+    <tr>
+        <td colspan="2">C</td>
+    </tr>
+</table>