added a basic camera

Added a camera class and unceremoniously put a global version in
me_GL_window.cpp.
just got the mouse tracking working

it took a fair amount of debugging, but the mouse tracking seems to be
working now.  Apparently it was working, but I had forgotten that I had
unbound a buffer, so when I tried to send updated data to the vertex
shader, there was nothing bound to GL_ARRAY_BUFFER.

Now I need to do some cleanup.
argh...now it isn't drawing at all

I'm going to post a link to this code repository on the openGL subreddit
and ask for help, so I need to commit.