-------------------------------------------
entry variable: $update -- frac proposed update
-------------------------------------------
default: 0.9

If "mo" ($momentumflag) is ticked, the "upd:"
entry (F3 panel, bottom) sets the fraction of the
CURRENT proposed movement to include in the
current update vector (tcl var: $update).

This affects the following functions:

  shrink
  area_shrink
  shrink2d
  curv_shrink_to_fill
  sphere_shrink
  ellipsoid_shrink
  ellipsoid_morph
  sphere_morph

Here is example code from area_shrink():

  /* final proposed movement this vertex */
  dx = tx*wt + ax*wa + sx*ws + nx*nc*wn + nx*nforce*wc;
  dy = ty*wt + ay*wa + sy*ws + ny*nc*wn + ny*nforce*wc;
  dz = tz*wt + az*wa + sz*ws + nz*nc*wn + nz*nforce*wc;

  if (momentumflag) {
    dx = decay*v->mx + update*dx;
    dy = decay*v->my + update*dy;
    dz = decay*v->mz + update*dz;
  }

  /* clamp move to 1 mm */
  d = sqrt(dx*dx + dy*dy + dz*dz);
  if (d>1.0) {nclip++; dx/=d; dy/=d; dz/=d;}
