Michael's tab work made GL initialization synchronous, so the UI can always assume it's initialized.  We should:

- Remove all remaining artifacts of gl_initialized_ (eg. from the background element)
- Have VrShell supply a callback to VrShellGL to grab the surface (not give it a pointer on which to call a public method).

etc.
 

Michael's tab work posted a CL that made GL initialization synchronous, so the UI can always assume it's initialized.  The CL was temporarily abandoned, but we should still land the GL init rework because it simplifies the UI startup.

Basically, instead of the UI starting, and later being told about its GL context, the UI thread will simply block until the context is ready, and can then proceed to initialize.  Then we can:

- Have VrShell supply a callback to VrShellGL to grab the surface (not give it a pointer on which to call a public method).
- Remove all remaining artifacts of gl_initialized_ (eg. from the background element)