OpenVR SDK 1.0.8:

General:
- Added VRCompositorError_InvalidBounds - This is returned when the application passes texture bounds to Submit that are outside the range of 0.0 to 1.0 or are otherwise invalid.          
- Added sample programs: hellovr_vulkan and hellovr_dx12.

IVRVirtualDisplay:
- The IVRVirtualDisplay interface is provided to allow OpenVR driver authors access to the final composited backbuffer intended for the headset’s display. The primary expected use case is for wireless transport, though this could also be used for saving output to disk or streaming. From the perspective of the runtime, the VR compositor is interfacing with a virtual rather than an actual display. See https://github.com/ValveSoftware/virtual_display

IVRSystem:
- GetOutputDevice interface added for applications to know which primary graphics adapter to use.  This returns a LUID on Win32, vk::PhysicalDevice for Vulkan and id<MTLDevice> on OSX.

IVRDriverManager:
- Public API to query installed device drivers. 

Drivers: 
- Drivers can now set Prop_DriverDirectModeSendsVsyncEvents_Bool to true to indicate they will call VsyncEvent on their own.  This enables avoiding the hardcoded 2.8ms vsync offset for IVRDriverDirectModeComponent implementations.
- Allow drivers to specify which graphics adapter to use by settings Prop_GraphicsAdapterLuid_Uint64.  This is a LUID on Win32.

MacOS/OSX:
 - includes a private framework compatible with Xcode 8 and higher. Embedding frameworks as binaries in your macOS app bundle can be error prone - see detailed instructions in issue #543 https://github.com/ValveSoftware/openvr/issues/543#issuecomment-307637564

Cmake Build System (optional):
 - cmake script changes to better support Cygwin
 - supports building universal 32/64 binaries on OSX
 - supports building as a private OSX framework

[git-p4: depot-paths = "//vr/steamvr/sdk_release/": change = 4009583]