* 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.
Differential Revision: https://developer.blender.org/D12626
