Support instancing about twgl.js

[greggman]

A big issue is how opinionated twgl should be here. I feel like it might be best if instancing only worked with vertex array objects. Why? Because if you don't use vertex array objects then twgl would have to reset the divisor on all attributes. Given vertex array objects are supported almost everywhere (pretty much only IE11) it would be best not to over complicate twgl to handle the case when not using vertex array objects?

OR? Maybe it doesn't matter. TWGL should just call vertexAttribDivisor if you specify a divisor on the attribute and it's up to you to deal with the consequences if you don't use a vertex array object. In other words you'd need to put divisor: 0 if you want twgl to reset the divisor for other draw calls when not using vertex array objects.

The other issue which I don't have a good solution for is how to handle the extension in WebGL1. You need to somehow tell twgl about the extension. Currently twgl is almost completely stateless. Except for a few defaults. But, to use the ANGLE_instanced_arrays extension requires either

1. twgl look it up every time on use

   that would seem like it would be too slow

2. have twgl require you to register the extension with twgl

   problem then is you can't support 2 contexts unless you manually register
   the correct context before expecting twgl to use it

3. have twgl add it to the WebGLRenderingContext.

   maybe something like

     twgl.setDefault({ addExtensionsOnContext: true })
   

   In which case TWGL would just copy as many extensions as it can/supports
   directly to the WebGLRenderingContext under their WebGL2 names. For example
   if you'd be able to use gl.createVertexArray instead of extension.createVertexArrayOES

   twgl would then use those features if enabled or maybe that should just be the default
   and if you don't want that you use

     twgl.setDefault({ addExtensionsOnContext: false })
   

I guess I'm leaning toward #3. It would mean any compatible WebGL2 code would just work

[greggman]

I ended up doing # 3 above and added the function twgl.addExtensionsToContext which you can call to manually enable and add all the extensions. I then added support for instancing. I'll try to whip up an example before I push it

[greggman]

okay I checked in 3.6.0 that supports instancing

https://github.com/greggman/twgl.js/releases/tag/v3.6.0

And also an instancing sample

http://twgljs.org/examples/instancing.html