More unit tests! And tests performance improvements

With the code coverage, I noticed that some code branches are hit a few million times (1,614,234,657 times in `FastBitmap.GetPixel()` and 133,959,185 times in `SetPixel()`).  
This is clearly ridiculous, so I reduced the number of images in the `Images` folder, so that it takes less time.

Note that it may also be a "code smell", maybe the bitmaps are actually read and written to too much. Only you can know, I don't understand a thing about the convolution, gaussian, octree etc.

On my computer, the unit test time goes down from about 2 minutes to about 50 seconds, and the coverage time goes down from about 40 to about 11 minutes.

Note that as long as any image operation hits so many things so many times, the coverage will still take forever.