All extensions are disabled when the WebGLRendering context is lost, with
the exception of WEBGL_lose_context. Include this in the section on
extensions, as well as the steps to perform in the case of a lost context.

Also update the WEBGL_lose_context extension spec to point out that
it is an exception to the rule.
Disabled extensions mark any WebGLObject instances they create as
invalid. Calling any method on a disabled extension generates
a INVALID_OPERATION error.
Merge pull request #334 from grorg/extensions-disconnected

Clarify the behavior of extensions if the WebGL rendering context is lost.