OpenVR SDK 0.9.20:
General:
* Added VREvent_InputFocusChanged - This event is sent whenever input focus changes from one process to another.
* Added VREvent_AudioSettingsHaveChanged - This event is sent whenever the user changes audio settings.
* Added VREvent_ReprojectionSettingHasChanged - This event is sent when the user turns interleaved reprojection on or off.
* Added VRInitError_Compositor_FirmwareRequiresUpdate - This error code will be returned by the compositor when there is a mandatory firmware update pending.

IVRCompositor:
* Compositor_FrameTiming now contains m_nReprojectionFlags. This will be 0 or more of:
  . VRCompositor_ReprojectionReason_Cpu
  . VRCompositor_ReprojectionReason_Gpu
* Added ForceReconnectProcess. Call this to force a disconnect and reconnect to the compositor.
* Added SuspendRendering. Call this when you know your process isn't going to render frames for a while. This often happens while loading new maps.

IVROverlay:
* Added overlay flag VROverlayFlags_TransferOwnershipToInternalProcess. This flag causes overlays owned by one process to be transferred to the new process spawned by a call to IVRApplications::LaunchInternalProcess. This can be useful for smoothing over such transitions from the user's point of view.

Driver Interface:
* Added VREvent_DriverRequestedQuit - A driver can post this event to ask the system to shut down.
* Added IVRDisplayComponent:: GetNextSwapTextureSetIndex to retrieve the index in the texture set to use for the next frame's render targets.
* Changed IVRDisplayComponet:: CreateSwapTextureSet to create three textures to allow a slightly longer round-robin list.

NOTE: As of SDK 0.9.20 the driver interfaces will remain backward-compatible indefinitely. Future runtime changes will not break the ability to load drivers built against at least SDK 0.9.20.

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 3426544]