Support 'oblicuboidsGrob()' / 'grid.oblicuboids()'?

[trevorld]

• {isocuboids} does some cool things with cuboids (instead of cubes) in an isometric projection
• Should we support functions that draw cuboids in an oblique projection?
• Similar arguments to oblicubesGrob() and grid.oblicubes() except the z value would be interpreted as the height of the cuboids.
• In the cuboid case shouldn't need to cull anything. May however need to tweak the sorting algorithm to only sort on (x,y) depending on the angle