Make gl2.New take self-referential option arguments.

[slimsag]

Right now gl2.New takes a single keepState bool argument. This doesn't allow for expansion in the future. We should use Rob Pike's design:

Self-referential functions and the design of options.

gl2.New(false) would become just gl2.New()

and

gl2.New(true) would become gl2.New(gl2.KeepState())