• Split GPUDisplay into two classes. PathTraceDisplay to implement the Cycles side, and DisplayDriver to implement the host application side. The DisplayDriver is now a fully abstract base class, embedded in the PathTraceDisplay.
• Move copy_pixels_to_texture implementation out of the host side into the Cycles side, since it can be implemented in terms of the texture buffer mapping.
• Move definition of DeviceGraphicsInteropDestination into display driver header, so that we do not need to expose private device headers in the public API.
• Add more detailed comments about how the DisplayDriver should be implemented.

The "driver" terminology might not be obvious, but is also used in other renderers.