Proposal: Let OpenGL depend on the StateVar package

[Sven Panne]

Probably not very surprising, I'm +1 on disentangling OpenGL and StateVar. :-)

Regarding the actual package name and the module name: I don't feel
very strongly about that, although inventing yet another
package/module name (it would be the third one!) might confuse people
even more. There is the StateVar package on Hackage already, and we
should have a very good reason to ignore that fact. "I don't like that
name" doesn't count IMHO, because this is a personal opinion and
doesn't outweigh the confusion introduced by yet another naming
scheme. Nevertheless, if that's the price one has to pay to get it
into the Haskell Platform, I won't object.

Regarding the actual content: I recently added a Functor instance for
GettableStateVar
(https://github.com/haskell-opengl/OpenGL/commit/3826a63d), but
instances for Applicative and Monad should probably be added, too.
WDYT?