Uncouple GL objects from Worker

[kkaefer]

We're currently creating objects that may initialize GL objects in the worker thread; they are part of the *Bucket objects (VAO objects, Buffer objects). Instead, we should only create the block of memory and completely defer creating/destroying the GL-related objects (wrapping GL Buffer object + VAO) to the map thread.

This should avoid issues like #1309

[jfirebaugh]

This is basically the same as #926 I think.

[kkaefer]

Yes