[css-masking-1] Re-allow specifying fill before other mask-border-slice component values

[cdoublev]

Moved from w3c/fxtf-drafts#530.

This PR re-allows specifying fill before the other component values of mask-border-slice, like in border-image-slice.

This is not allowed since this commit, but I think it was unintentional.

Basically, this commit moved the property definition table of mask-box-image-slice from masking/Masking.src.html:

<tr>
  <th>Name:
  <td><dfn>mask-box-image-slice</dfn>
<tr>
  <th><a href="#values">Value</a>:
  <td>[<var>&lt;number&gt;</var> | <var>&lt;percentage&gt;</var>]{1,4} &amp;&amp; fill?

... to masking/Overview.src.html:

Name: mask-box-image-slice
Value: [<<number>> | <<percentage>>]{1,4} fill?

An later mask-box-image-slice was renamed to mask-border-slice.