This PR adds the possibility to overlay rectangular grids on 3D surfaces as provided by GLSurfacePlotItem. It also extends the current example of 3D surface plots to showcase the rectangular grids.

Here's a screenshot of the updated example:

The current implementation already features adding a wireframe to a 3D surface plot, but unfortunately the grid lines don't look nice and it's only possible to show the actual triangle mesh wireframes and not rectangular grids, which IMO is useful to visualize and analyze data.

To nicely overlay the grid, the only feasible I could find, is to create additional GLLinePlotItems that are added to the scene. To have this working, I added the _setView method, to make sure the view is available. As soon as this is the case, the grid lines are added to the scene. For proper display, certain OpenGL options need be set for the GLMeshItem and GLLinePlotItem.

Not sure if there would be a better way to handle this, but at least this seems to do the trick. I am open for suggestions for improvements or alternatives to this approach.

Also note, that adding rectangular grids to animated surfaces imposes a serious performance drop. Because of this, I didn't add the grid to animated surface in the example. Maybe we should a note about this somewhere, although I am note sure where. (With the latest changes, this doesn't seem to be an issue anymore)