Make lambda used for motion estimation RD adaptive.

Previously this was a hard-coded constant (chosen just to get
 something working).
This uses the "theoretically optimal" value (with a cap), which may
 not actually be very good, especially since the rate model we use
 in mcenc.c is highly inaccurate and we don't consider during the
 motion search whether or not PVQ's noref flags will throw away our
 prediction.