Port @rougier's implementation of the collections system in glumpy.

The collections system allows us to create a collectable visual. Multiple visual instances (items) of the same type can be collected together and rendered in a single pass. OpenGL variables can be local (one value per vertex ==> attributes), shared (one value per item ==> uniform packing/unpacking in textures), or global (one value for all items ==> regular uniforms).

This system is critical for performance in vispy.scene.