Add gimp_drawable_gradient_adjust_coords(), which adjusts the
gradient segment coords according to the gradient type, so that, in
cases where the gradient span is unrelated to the segment length,
the gradient cache (in GimpOperationGradient) is big enough not to
produce banding.  Use the new function in gimp_drawable_gradient()
and in the gradient tool, instead of duplicating the logic.

Move the shapreburst coordinate-adjustment logic to the new
function, and add appropriate logic for conical gradients.

Remove the code that avoids using the gradient cache for conical
gradients from GimpOperationGradient.