Tidy the GLES state block

[ben-clayton]

Add a global for CurrentState so we don't have to always go though the map!(ThreadID, ref!Context) Contexts global at the root of the state tree (ensure you update all Link() logic in gles/links.go).

Find a way to hide GLXContexts, WGLContexts, CGLContexts, EGLContexts.

Try navigating the state block in GAPIC, make changes to the API file to try and make state navigation not suck.

[dsrbecky]

+1 for the global. Do we want to introduce the concept of thread-local storage, or we just update it before each command?

UI wise: How about moving all globals to combo box?

[ben-clayton]

Given that the state reflects the currently selected command, then the state should probably reflect the current thread of that command.