Don't create GL objects in the tile worker thread

[kkaefer]

We should check that we're not creating any GL objects or calling any GL functions in general in anything but the Map thread.

[ljbade]

Hmm, easiest way is it look at the attach/detach functions since any GL calls in a thread without a GL context bound will throw and exception.

[kkaefer]

FWIW, I don't think we're currently doing this, but we should do a proper review to avoid any undefined effects later on.

[ljbade]

Another trick would be to add a thread ID check to the GL function error check wrappers.

[jfirebaugh]

Creation of GL objects now requires a GLObjectStore, which the worker thread does not have access to, so we can be fairly certain this does not happen.