Function lerp2

Bilinear interpolation between a1, b1, a2 and b2 based on s and t, where s and t are numbers between 0.0 and 1.0.

The calculation is as follows: a1 and b1 are interpolated based on s to give p, a2 and b2 are interpolated based on s to give q, and then the final result is obtained by interpolating p and q based on t.

The result will be equal to a1 when both s and t is 0.0, equal to a2 when s is 0.0 and t is 1.0, equal to b1 when s is 1.0 and t is 0.0, and equal to b2 when both s and t is 1.0