make interpolate_* public

[Lishen1]

make it public. It's usefull functions
interpolate_nearest(image, x, y, default);
interpolate_bilinear(image, x, y, default);
interpolate_bicubic(image, x, y, default);

[nan-mu]

Hello, I'm in the process of tidying up some functions to be made public on #544. I've looked into the three interpolation functions you mentioned, but it seems they expect to be called through warp_into and wrap_into_with. Is there another purpose for them?

[Lishen1]

Just to access pixel value with real coordinates