Move functions requiring wide dependencies to their own files.

Created by: yohcop

This makes it easier to use gonum/plot without having to install all of the dependencies if one does not care about them. For example, if tiff, pdf, eps and x11 output are not required, it is easy to delete the vg/vgimg/tiff.go file, vg/draw/formattedcanvas.go (depends on all the formats known to plot), and the directories vg/vgpdf, vg/vgeps and vg/vgx11.

I left pdf and jpg in vg/vgimg/vgimg.go, since they do not require a third party library.