message: metainfo.xml: Adds missing changelog entry for Sixel aspect ratio fix.
diff: <description>
  <ul>
  <li>Fixes a problem with oversized glyphs being wrongly cut off (#821).</li>
  + <li>Fixes Sixel handling of non-standard aspect ratio (#827).</li>
  <li>Fixes Sixel mode, when updating the color palette with a new color, that color must also be used for subsequent paints.</li>
  <li>Fixes vertical cursor movement for Sixel graphics with only newlines (#822).</li>
  <li>Fixes Sixel rendering for images with aspect ratios other than 1:1.</li>
  