Instead of making the extension objects request the OpenGL extensions
themselves, we can do it here since we already store that information
to be able to compute the list of available extensions. As bonus points
this makes it impossible to forget to request an OpenGL extension when
implementing a new WebGL one.
Before this change, we were going through the chain of base classes for
each IDL interface object and having them set the prototype to their
prototype.
Instead of doing that, reorder things so that we set the right prototype
immediately in Foo::initialize(), and then don't bother in all the base
class overrides.
This knocks off a ~1% profile item on Speedometer 3.
