Idea: Optimize gradient <stop>s

[silverwind]

In some cases, adjacent gradient <stop>s with the same color value can be safely removed:

<linearGradient>
  <stop offset=".1" stop-color="red"/> <!-- can be removed -->
  <stop offset=".2" stop-color="red"/> <!-- can be removed -->
  <stop offset=".3" stop-color="red"/>
  <stop offset=".5" stop-color="blue"/>
  <stop offset=".6" stop-color="blue"/> <!-- can be removed -->
  <stop offset=".7" stop-color="blue"/>
  <stop offset=".8" stop-color="green"/>
  <stop offset=".9" stop-color="green"/> <!-- can be removed -->
</linearGradient>