Multiple viewports do not render correctly

[McManning]

• Splitting a viewport will cause the other viewports to not update to a render texture from Unity
• Unity is tracking the cameras correctly per-viewport.

[McManning]

Fixed with the fix to #4. For the multiple viewports issue, the buffer node count is 1 and each viewport tried to write to the buffer every ViewportController.Update call - but the "first" always won so other viewports couldn't write.

I changed the behaviour to instead step through the list of viewports in SyncManager and ask each one in order for their RT each update. So there's definitely slowdown with the more viewports you have - and there should be some form of prioritization for the "active" viewport in Blender. But that's a future enhancement.