OpenVRHMD safe shutdown

[smlpt]

This PR adds initalized = false to OpenVRHMD.close() before calling the shutdown logic to prevent the renderer from accessing memory that doesn't exist anymore.

[skalarproduktraum]

@smlpt While you are at it, would you be so kind to check the other functions in OpenVRHMD as well?

[smlpt]

@smlpt While you are at it, would you be so kind to check the other functions in OpenVRHMD as well?

Except for various non-nullable methods that access HMD properties, nothing comes to mind.
You mentioned previously you'd like to eventually re-think the whole shutdown logic, but I think that's a story for another PR.