extract_texture(...) overloads for Image3u and Image4u

[patrikhuber]

Currently, some of the functions of the texture extraction expect Image4u, like create_mipmapped_texture, and sometimes Image3u is used. This could be made more consistent and perhaps we want to use Imageu4 everywhere. Or perhaps we want to make texture extraction overloads for both 3-channel and 4-channel input images.

Related: #308