Merges the GLFW-based embedding implementations for Windows and
Linux into a single shared implementation. At this point the implementations
are largely identical, and having two copies is increasingly a maintenance
burden.

Originally they were separate since they were intended to be throw-away,
and their development proceeded at different speeds. Now that they have
reached parity, and there's a possibility that the GLFW implementation
will be longer-lived than initially expected, having one copy that's testable
on several platforms should simplify development.