Fix OpenGL shader/texture sharing on PySide

For some reason, the `GLViewWidget.ShareWidget` class attribute stayed as `None` on PySide. That is, `ShareWidget` was created, but other instances of the `GLViewWidget` class just saw `None`. It works great on PyQt4.

This may be to a difference in the way PySide and PyQt4 bind `QtOpenGL.QGLWidget`. Maybe PySide is using some metaclass mumbo jumbo that erases class attributes.

I simply made `ShareWidget` a module-level declaration. I see no problem with this, since the shaders are themselves declared at the module level.

See [this thread](https://groups.google.com/forum/#!topic/pyqtgraph/AOj_oRoN_fk) on the mailing list for more information.