allow texture map of a raw array

[mdsumner]

We could default to [0,1, 0, 1] but more generally the underlying mapping functions should accept an array, raster-palette, or raster-RGB (and allow set xmin,xmax,ymin,ymax) - see FIXME in texture_mapping

[mdsumner]

Notes from #89

not sure about these

• mesh_plot() should not include crs and coords args
• as.mesh3d() should NOT accept coords or crs we just force use of copy_down()
• all reprojection should be done with reproj() (which means mesh3d needs crs, or we have a reproj_to_mesh() workflow, which might be the go)

[mdsumner]

Note that the way Textures work is now documented:

https://github.com/rforge/rgl/blob/master/pkg/rgl/vignettes/rgl.Rmd

e.g.

• The color in the figure above was specified to be white. By default,
  the colors in the bitmap will modify the colour of the
  quad. If col is black (a common default), you won't see anything.
• On the other hand, you usually don't want specular
  reflections (which show up as glare). Setting specular
  to black prevents those.

w00t