Use wxUIntPtr rather than (unsigned) long in wxImage::ResampleNearest() as long is still 32 bits under Win64 and so doesn't allow the code there to work with images larger than 2^16 in either direction, when it could be allowed in this case. Document the current limits on the size of the image and add a unit test checking that resizing images of size greater than 2^16 works in 64 bits. See #18550.