Add SetScaleStyle for zoomed images

[gonutz]

My use-case is to create an educational video where I want to explain an algorithm that works on images.

I would like to start the video by zooming into an image until we can see the pixels. However, in gg I get a blurry image when zooming in.

By default gg uses the draw package's BiLinear interpolation. This PR adds SetScaleStyle which I use to adjust the interpolation dynamically. While the image in my video is zoomed out, it is linearly interpolated. But once the zoom level is > 1, I switch to NearestNeighbor to show the pixels.

I have tried to keep this in line with the rest of the code, adding SetScaleStyle with a parameter and concrete SetScaleBiLinear etc. for easier auto-completion (if that is your reasoning, makes sense to me at least).