Use a more reliable way to detect modal rendering.
Use a more reliable way to detect modal rendering.

It can happen that the UseFastDraw function is called before
the ViewportRenderEngine is fully initialised. This means that
_cycles is effectively still null when the UseFastDraw is
already called.

Instead use a boolean, since the information is already known
at the start of a render.

mcneel/RhinoCycles