Effects > Artistic > Relief: complete overhaul

Dialog added, three selectable params (angle, thickness, depth), whole
algorithm reworked.  Basically this is now very close to the
emboss/engrave filter, but instead of using a color as the base, the
existing image data is used, and the relief effect is only applied to
luminance data.  This gives the image a pseudo-3D raised or sunken look,
without corrupting the existing color balance.