Resize for arbitrary implementations of ImgBase

[hageldave]

Resize for Img can be done using the g2d mapping, but this may not work for other implementations of ImgBase (maybe not even for ColorImg) especially when creategraphics is not supported.
The approach for downsampling should use mipmaps or image pyramids of some kind, and interpolate between pyramid layers for sizes in between halves to counteract aliasing (trilinear interpolation).
For upsampling, linear interpolation may suffice, but if not too complicated, 2nd or 3rd order spline surfaces can be used.