Feat: Extend platform abstraction

[wouterlucas]

Move Image creation and GL wrapper to Platform.

This PR changes:

• createImageBitmap support to Platform
• Image fetch to Platform (by default using XMLHttpRequest)
• ImageWorker handling to Platform
• GLWrapper to Platform
• Similar, support for TX compression and SVG handling has been moved to Platform

Removed:

• createImageBitmap auto detection - this needs to be done by Platform
• Image Fallback handling - this is done by Platform

NOTE: BREAKING CHANGES
This requires a different Platform import / init to make it work.

The concept is you create a Platform that matches the functionality the platform exposes. I.e. if the Platform does not support createImageBitmap or a less version of createImageBitmap you should extend the platform to match that.

@estobbart for ION you can create or upstream an ION specific platform (or we can create one) that:

• Replace loadImage and createImage in Platform (possibly Fetch too?)
• Replace the respective GL function

Todo:

• Expose platforms in exports
• Create Legacy Platform for non-createImageBitmap platform
• Create Chrome 50 createImageBitmap platform
• Create fetch supported platform

[estobbart]

Couple of questions..
L2 had a force draw capability so that every raf would draw, is that possible here?