Add shrink factor to reduce computation

[luigi311]

Add a shrink factor argument to reduce the computation of the mapping calculations. For most things you dont need super high resolution for mapping images together so instead create a copy of the images at a shrunk down ratio/resolution, perform the orb/ecc match, scale up the warp matrix to match the original images and then do the image stacking on the original images. This should provide greater performance the higher the resolution of the images. You can see my original python implementation here as a reference, also built from the same python source you used.

https://github.com/luigi311/Low-Power-Image-Processing/blob/master/stacking/stacking.py