Merging using vr_steamvr_rel_to_sdk_release to create OpenVR SDK v1.2.10

General:
 * Added new, more specific error messages for many compositor startup failures.
 * Fix for IVRCompositor.GetFrameTimings C# binding (https://github.com/ValveSoftware/openvr/issues/1001)

Event and Overlay changes:
 * Scroll events have been split into two types. The first, VREvent_ScrollDiscrete, is meant for applications that are tuned to accept primarily detented mousewheel events, which replaces the previous VREvent_Scroll event. The second, VREvent_ScrollSmooth, is for applications that are tuned for more touchscreen-like, analog scrolling.
 * VROverlayFlags have been updated to allow overlays to indicate their preferred scroll event type. The VROverlayFlags_SendVRDiscreteScrollEvents flag renames the VROverlayFlags_SendVRScrollEvents flag, and the overlay will receive VREvent_ScrollDiscrete events when this flag is set. The VROverlayFlags_SendVRSmoothScrollEvents flag is added, and the overlay's owning application will receive VREvent_ScrollSmooth events when this flag is set.
 * VREvent_Input_BindingsUpdated is sent when a user has updated controller bindings using the system input binding UI.

IVRIOBuffer:
 * Adds ability for writers to detect if an IOBuffer has readers (IVRIOBuffer::HasReaders), to avoid potentially expensive writing work

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 4924548]