Add --paste-scale parameter to adjust pad sizes in paste layer

This parameter changes how pads in the paste layer are drawn,
generally so that they can be made smaller to account for tool width
when cutting a stencil.

It should probably be a dimension instead of a scale factor, but this
hack got my stencils made.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: bert <bert.timmerman@xs4all.nl>