Recalculate weights only on block boundaries

[mjethani]

Hi, this is an optimization.

bmvbhash() recalculates y_mod, y_frac, y_int, weight_top, and weight_bottom and x_mod, x_frac, x_int, weight_left, and weight_right on every iteration of the outer and inner loops respectively. These variables need to be updated only when the loops cross block boundaries.

In this patch we keep track of block boundaries using two new variables y_mult and x_mult.

This reduces the number of times the weights must be recalculated from width * height to bits ** 2.