Previously, the LibWeb bindings generator would output multiple per
interface files like Prototype/Constructor/Namespace/GlobalMixin
depending on the contents of that IDL file.
This complicates the build system as it means that it does not know
what files will be generated without knowledge of the contents of that
IDL file.
Instead, for each IDL file only generate a single Bindings/<IDLFile>.h
and Bindings/<IDLFile>.cpp.
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.
