Significant performance improvement of `alpha_composite` function.
approximate test for lossy image writing
py3 as well
JpegImagePlugin sets bufsize for optimized images
Precision improvement. ≈60% performance lost
speedup. divisions replaced by shifts.
Merge pull request #158 from wiredfool/maxblock

JpegImagePlugin sets bufsize for optimized images
Merge pull request #157 from wiredfool/webp_tests

approximate test for lossy image writing
Merge pull request #156 from homm/master

Performance improvement of alpha_composite function
Precision improvement. No performance affected.
Merge pull request #159 from homm/master

Precision of alpha composite. Again.