Intro lessons

I sketched out the first two intro lessons. There are two outstanding issues:

* I am not quite sure how we are going to do assessments.  I am assuming that per-pixel diffs should be ok, but maybe I will wait until we get individual contexts per exercises
* Second, the drawing buffer sizes for the lessons don't match the frame buffer sizes which messes up the scissor test example.

Also looking ahead, we may want to do something like shim parts of the code for some purposes.  For example, it might be ok to do a bit of magic/behind the scenes hand holding to prevent students from getting overwhelmed when we start introducing buffers (which is the next step in this process).  

Part of the problem is that there is a big step going from "clear the screen" to "draw a triangle" which requires at minimum:

* Creating and linking a shader program
* Creating and initializing a vertex buffer
* Setting all vertex attributes and locations
* Binding the shader and vertex buffer
* Calling a draw command

If we hit them up front with all of this it is going to be too much.  So maybe the way to go is to slowly split these into pieces with some extra code / magic around their test shim to help them ease into things.