Remove dependency on ImageMagick

VISE depends on ImageMagick for the following purpose.

1. read and write images (jpg, png)
2. query colour space (RGB, Gray, etc.) of an image, convert if it is not RGB
3. initialise an image from its BLOB representation (i.e. binary representation stored in a string variable)
4. transform an image based on an affine transformation matrix (H = [a, b, c, d, e, f, 0, 0, 1])

[1] can be obtained by the self contained, single header library stb_image.h. I have used this library in other projects and it works remarkably well.

If we can find a simpler way to do [2], [3] and [4], we can remove a big dependency of VISE. Furthermore, this will help cleanup a lot of code (e.g. #ifdef RR_MAGICK). I do not yet know what is the best way to address [2], [3], [4]. We are open to advice and help.