The `overflow` property doesn't apply to table track and track groups,
and table elements only accept a few `overflow` values.
Therefore, this patch adds an `effective_overflow()` method to get the
actual value that needs to be used.
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>
For example:
```html
<table border="1">
<tr> <td></td> <td></td> </tr>
<tr> <td colspan="2"></td> </tr>
</table>
```
We should initially size the columns according to the cells in the first
row since they have a span of 1. Then we handle the cell in the second
row with a span of 2, this should be able to increase the size of the
columns, but never decrease them.
