Bug: dmabuf patch hangs after a couple minutes

[mrxz]

It seems that the dmabuf approach can freeze up (crash?) after a while. There doesn't seem to be any rhyme or reason to it. Probably a race condition or synchronization issue. Honestly not too surprising given the hacky nature of the patches, but it's worth investigating if it can be resolved.

From my (limited) testing:

• The WebXR experience freezes (no full crash, nor error in logs)
• Occurs anywhere between a couple minutes to 1 hour (haven't made it past an hour since)
• Didn't seem to occur at the time the patch was written (either just luck, or changes in FF code base, SteamVR, drivers, etc... might have surfaced the issue)

Also reported by #1 (comment)

[kilves]

@mrxz I believe I've found the issue. There's a synchronization problem while copying memory in VRShMem::PushBrowserState/PullBrowserState where geckoGenerationA/B flags go out of sync and lock up the render thread. Solution is as simple as adding memory locks. I'll post a PR when I can