Clean the activation process of gloo objects

Basically, GLObject only provides _activate, and all objects now keep
track of _need_something, which they then "do" in the _activate method.
This may add a wee bit more code in some classes, but now at least it
is very clear what happens. This was especially problematic for the
Program class.
small fix for last commit
flake
updates to gloo from scene
Merge pull request #307 from almarklein/gloo-clean

Gloo clean