Spacing of matrices

[tmke8]

• Spacing needs to be set to 0 at the left and right edges
• investigate whether matrices have a fixed grid
• in any case, increase horizontal spacing a bit

This, for example, looks correct (in Chrome, which is the most spec-compliant in this area):

<math>
  <mrow>
    <mo>|</mo>
      <mtable displaystyle="false" scriptlevel="0">
      <mtr>
        <mtd style="padding-left: 0; padding-right: 0.5em;"><mi>a</mi></mtd>
        <mtd style="padding-right: 0; padding-left: 0.5em;"><mi>b</mi></mtd>
      </mtr><mtr>
        <mtd style="padding-left: 0; padding-right: 0.5em;"><mi>c</mi></mtd>
        <mtd style="padding-right: 0; padding-left: 0.5em;"><mi>d</mi></mtd>
      </mtr>
    </mtable>
    <mo>|</mo>
  </mrow>
</math>