Upgrade to OpenTK 3/4

[neokabuto]

In my usual genius, I finally updated all the tutorials... to OpenTK 2. 3 has been out for a while and 4 is coming soon.

[neokabuto]

My new decision is that OpenTK 4 is still going to be months after already waiting so long, and the changes required are relatively small, so I'll go ahead and do it.