Add information on planned backwards incompatibility in WebGL 2

It was decided in the WebGL WG F2F that WebGL 2 will not be entirely
backwards compatible with WebGL 1. In particular, there is agreement
that all extensions are not moving over, and always enabling primitive
restart in WebGL 2 significantly streamlines implementations, so that
could be added to this section of the spec.

Fixes small issues in the extension registry.
It's a base interface for both WebGL1 and WebGL2. It's not something that developers will ever see, almost an implementation detail, but defining the interfaces as such allows the spec to be more concise.

The inheritance tree looks like:
WebGLRenderingContext : implements WebGLRenderingContextBase
WebGL2RenderingContext : implements WebGL2RenderingContextBase, implements WebGLRenderingContextBase

A theoretical WebGL3 may inherit from the bases for WebGL1, 2, and 3, assuming it's intended to be backwards compatible in some capacity.