This adds two methods, write_texels and read_texels, to the Image class.
Conversion between image formats happens automatically. The layout of
the client image data is passed in via ImageDataLayout struct.
This serves as the storage for all image types. 1D, 2D, 3D, Cube and
image arrays.
Upon construction a full mipmap chain is generated and the image is
immutable afterwards with respect to its layout.
