Allow the user to choose the two anchor frames to use for linear interpolation.

[torque]

In some cases, the first and last frames of an animation may not be the best frames to use to calculate the starting and ending position of a linear transform. There are a couple of complications that arise when considering implementing this:

• There is no good place in the user interface to stick two more number edit fields that will be completely useless most of the time.
• Should this allow applying linear motion to segments that are longer than the tracking data? This could potentially result in unintuitive behavior.