Consider exporting shapeGrob

I'm not sure if ggforce would be the right place for it, but the `shapeGrob` function behind `geom_shape` might be useful for me, and perhaps for others who want the same functionality. My use case is for https://github.com/hughjonesd/ggmagnify, to allow a nice rounded outline for the magnified area. Using GeomShape directly wouldn't fit my needs, though I could hack it.