Optimize wxImage::Scale() by pre-calculating the values used.

Optimize wxImage::Scale() by pre-calculating the values used.

Precompute the values used by various resampling algorithms once instead of
doing it for each pixel. This dramatically speeds them up: more than 3 times
faster in wxImage benchmark with the small images and up to 5 times faster
with larger images.

Closes #15281.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74321 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775