Split image & add example

[QuanticPotatoes]

I propose to add the possibility to add an single image in a <a-sky> element.

Like this

And split it with the property split

<!-- Paths to left and right images -->
      <a-assets>
        <img id="both" src="textures/LR.jpg">
      </a-assets>

      <!-- Camera -->
      <a-entity camera look-controls position="0 0 0" stereocam="eye:left"></a-entity>

      <!-- Sky sphere -->
      <a-sky id="sky1" src="#both" stereo="eye:left;split:vertical"></a-sky>
      <a-sky id="sky2" src="#both" stereo="eye:right;split:vertical"></a-sky>

Have a nice day 😃 !

[QuanticPotatoes]

sorry, i have commit also the build files, my mistake

[vincentfretin]

I did a similar change to support Top/Bottom images 5 years ago, so about the same time.
I just did an alternative change in my current project, see #45