**OpenVR SDK 1.8.19**

Properties:
* Added Prop_DashboardScale_Float.  Each headset has the opportunity to provide a scale for 'comfortable dashboard UI legibility'.  For reference, Vive is 1.0 scale and Index is 0.75 scale.
* Added Prop_IpdUIRangeMinMeters_Float, Prop_IpdUIRangeMaxMeters_Float to control the IPD user interface range, in-headset.
* Added Prop_Audio_DefaultPlaybackDeviceId_String, Prop_Audio_DefaultRecordingDeviceId_String for HMDs to provide identifiers for associated audio playback and recording devices. On Windows these are Endpoint ID Strings.
* Added Prop_Audio_DefaultPlaybackDeviceVolume_Float for HMDs to specify a default volume level which will be set the first time the HMDs associated audio playback device is selected. Prop_Audio_DefaultPlaybackDeviceId_String must also be set. The volume range is 0 to 1.

IVRApplications:
* Added GetSceneApplicationState(), which returns the running application state ( None, Starting, Quitting, Running, etc).  See VREvent_SceneApplicationStateChanged
* Removed GetTransitionState()
* Removed IsQuitUserPromptRequested()

IVRSystem:
* Removed AcknowledgeQuit_UserPrompt().   "Prompt user on quit" functionality is no longer supported.

IVROverlay:
* Added VROverlayFlags_HideLaserIntersection - This causes the laser mouse to not draw the intersection blob for this overlay. The overlay is responsible for drawing its own cursor, if appropriate.
* Added support for rendering overlays as gravity aligned curved surfaces. See new Set/GetOverlayCurvature definitions for details.
* Removed gamepad support in overlays. This includes GetGamepadFocusOverlay(), SetGamepadFocusOverlay(), SetOverlayNeighbor(), and MoveGamepadFocusToNeighbor(). Going forward gamepads will be supported via lasermouse instead of with these custom calls. 

IVRInput:
* Added OpenBindingUI - This function opens the binding user interface.

Events:
* Added VREvent_SceneApplicationStateChanged
* Removed VREvent_SceneFocusLost, VREvent_SceneFocusGained, VREvent_SceneApplicationSecondaryRenderingStarted, VREvent_ApplicationTransition{*}
* Removed VREvent_QuitAborted_UserPrompt

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 5469045]