Fix for blurry images from WEBP input and small output dimensions

[totallyunknown]

This will fix blurry images if the input is in webp format and the output is relativly small in his dimensions. This will fix issues #220, #339 makes #222 obsolet. This will also fix h2non/imaginary#240.

The underlying issue is, that the factor comming out from supportsShrinkOnLoad is too high. transformImage has then a too large shrink and the image gets blurred.

I have no good idea, how to put that in a unit test, since the image output may differ on plattforms.

I did some manual testing with a lot of different output resolutions and compared the old with the new code. All blurring is gone.

shrinkOnload uses for WEBP now the same logic as for JPEG.

[colin-lee]

good job, that helps me. please merge it

[totallyunknown]

Looks like this project is abandoned...

[colin-lee]

bad news, there are so many PR waiting for merge。
I compile bimg with your patch, then update imaginary.
It fixed the blurry images.
many thx

[wuzhihui1123]

good job, that helps me. please merge it
@h2non