GLTFLoader: Examples reportedly failing in IE11

Per Stack Overflow (https://stackoverflow.com/q/52144252/1314762, https://stackoverflow.com/questions/51558365/how-to-load-gltf-scene-in-threejs-using-ie11), GLTFLoader is reportedly not working in IE11 even when the Promise polyfill is included. Our example pages do not include the polyfill at all, and therefore also do not work.

I think we should:

• Verify Promise polyfill is still the only thing needed to make GLTFLoader work in IE11
• If not, update either the loader or the docs.
• Add the promise polyfill to the existing glTF examples, with an <!--[if IE]> block, if we want them to work in IE11.

[Mugen87]

In this case I vote to close the issue since I’m not aware of that we’ve put polyfills in other examples. Besides, the comprehensive documentation page about GLTFLoader already notes the need for a polyfill in context of IE 11.

[vyv03354]

IE 10+ no longer supports conditional comments (such as <!–[if IE]>). IE 11 will ignore conditional comments just like other modern browsers.