Add instancing support for Bars3D.

QtDataVisualization's OptimizationStatic was only supported for Scatter3D, but instancing works in a different way and is usable for Bars3D too.

Note: We'll probably make instancing the OptimizationDefault for all graphs, add a new optimization for the old style (OptimizationLegacy, or something) and perhaps recreate the static one the same way it was done for QtDataVisualization.