SDK Release 0.9.21:

IVRApplications:
* Added VREvent_ApplicationListUpdated event. This is sent whenever an application manifest is added, removed, or updated.


IVRRenderModels:
* Added VREvent_ModelSkinSettingsHaveChanged event. This is sent when the user has selected a new controller or base station/camera replacement model. If your application uses the IVRRenderModel interface to load the user's actual controllers it should reload them after receiving this event.
* Added GetRenderModelOriginalPath function. This returns the non-overridden full path to the render model so controller/base station/camera skinning can be avoided if the application requires it.
* Added GetRenderModelThumbnailURL function. This returns a URL (usually to a file) for a 4x3 aspect ratio thumbnail image that can be used to preview the render model.
* Added GetRenderModelErrorNameFromEnum so applications don't need to provide their own lookup table to interpret these errors.


IVRCompositor:
* Added cumulative stats tracked per-application and associated accessor.
* Added interface to access the mirror textures (per-eye).  This is the undistorted view with chaperone and overlays (e.g. dashboard) drawn on top.
* Added preliminary support for screenshots. This interface is not stable yet and should be ignored for now.


IVRTrackedCamera:
* Exposed the initial version. Provides developers access to poll streaming frames from the HMD front facing camera with the associated tracked HMD pose.
The image is available as either a pre-corrected distorted view or corrected undistorted view. Added a thin Qt example for getting and showing the camera image/pose.


IVROverlay:
* Added interface to get size of a texture used by an overlay.  This is useful when setting the overlay texture by filename.
* Added ability to render side-by-side stereo content and panoramas.


Driver Interface:
* IVRCameraComponent is still in a development phase. 
* Added provider method GetInterfaceVersions. This contains all the version numbers of the other interfaces in the driver. Drivers should return vr::k_InterfaceVersions from the version of openvr_driver.h that they were compiled against.
* GetTrackedDeviceDriver and FindTrackedDeviceDriver no longer take an interface version. The caller will expect the version of the interface returned in GetInterfaceVesions()


HelloVR sample:
* Fixed compile error around glDebugMessageCallback that could happen with certain versions of the OpenGL headers.
* Fixed compile error from LoadRenderModel->LoadRenderModel_Async name change.
* Fixed compile error from LoadTexture->LoadTexture_Async name change.

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 3465769]