Cycles: Add support for native OptiX curve primitive

This patch adds support for the curve primitive from OptiX to Cycles. It's currently hidden
behind a debug option, since there can be some slight rendering differences still (because no
backface culling is performed and something seems off with endcaps). The curve primitive
was added with the OptiX 7.1 SDK and requires a r450 driver or newer, so this also updates
the codebase to be able to build with the new SDK.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8223

intern/cycles/blender/blender_python.cpp

@@ -92,6 +92,7 @@ bool debug_flags_sync_from_scene(BL::Scene b_scene)
   flags.cuda.split_kernel = get_boolean(cscene, "debug_use_cuda_split_kernel");
   /* Synchronize OptiX flags. */
   flags.optix.cuda_streams = get_int(cscene, "debug_optix_cuda_streams");
+  flags.optix.curves_api = get_boolean(cscene, "debug_optix_curves_api");
   /* Synchronize OpenCL device type. */
   switch (get_enum(cscene, "debug_opencl_device_type")) {
     case 0: