GC-92: Implement bilinear interpolation for Blend algorithm

[cesarsouza]

The image blending algorithm (Blend.cs) currently uses a nearest neighbor approach to merge images, which can sometimes result in pixelated results, specially for text applications. I would be interesting to add at least the possibility for bilinear interpolation in the filter options.

On a side note, more unit tests should be written for the Blend filter before this can be done.