Limit the number of lines drawn in the transform tool canvas grid to
one line every 5 image pixels. This should probably be done in display
pixels, didn't change that yet.
and get the shell from gimp_canvas_item_get_shell() if needed. Remove
some cairo_translate() that didn't have any effect and were leftovers
from a very early canvas item code.
which removes a lot of code from the transform tool, and reduces the
number of canvas items used for the transform grid from possibly
gazillions to one.
