bilinear filtering

Finally PRICE got proper Bilinear filtering.

This was surely long due! The previous linear h+v filter was little more than a quick hack.

Bilinear filtering is quick, good and quite sharp and works well with scaling between the 50% and the 150% range. It is quite good when downscaling while upscaling beyond a certain range surely has better algorithms.

LaternaMagica and its export tool with automatic constraining will benefit from this new filter too.