Event attributes trigger event twice in HTML encoding

[ksons]

Using eventattributes such as

  <mesh src="teapot.json" onclick="console.log('Teapot');"></mesh>

trigger twice. This affects only HTML encoding and event attributes. Using addEventListener or the same scene in XHTML are not concerned.

[csvurt]

I've added checks to the mouse event attributes polyfill that will prevent it from adding a second event listener to the element in HTML documents.