Effects > Artistic > Colored pencil: all-new implementation

PD's old "Pencil" effect was specifically called out as shitty in a
3rd-party review, so it was time to fix this tool once and for all.
Developments in 6.4's cycle - like the new compositor class - made this
a much easier task than it otherwise could have been.

Four pencil modes are now provided - normal, luminous, pastel, and
graphite - with support for variable tip thickness and pressure.  This
provides a wide variety of styles, with much better results than the old
method, and very good performance (even on photo-sized images).