animation: When fading, round the surface alpha when close to 0 or 1

animation: When fading, round the surface alpha when close to 0 or 1

The spring code stops when the current value is withing 0.0002 of the
target. In that case, round the value to 0.0 or 1.0 to enable the use
of fast paths, such as disabling blending in the GL renderer when an
opaque region is set.