textures as a copy_down() task

[mdsumner]

To be considered, from #90

This is actually a mode for copy_down, e.g.

• copy_down(x, BasicRaster) - put values(BasicRaster) on 'x$vb[3, ]' via x$vb[1:2, ]
• copy_down(x, RasterBrick) - put 3 layer brick values on $vb[1:3, ] (because we have coords+z independent of the mesh structure, like a climate model mesh)
• copy_down(x, texture = RGBBrick) - put texture(RGBBrick) on 'x$material$texture' via x$vb[1:2, ] and texcoords
• copy_down(x, texture = RGBBrick, cheat = TRUE) - put colours RGB on $material$color

Put some thought into the argument logic, copy_down(x, z = brick, texture = TRUE/FALSE) for example