aquabsd.alps.raster: New device for image loading/rendering

[obiwac]

Currently, this is handled by aquabsd.alps.png, which is format-specific. If we were to go on to create separate aquabsd.alps.bmp, aquabsd.alps.jpeg, &c devices, the client would have to handle all these file formats itself (which goes against AQUA's device philosophy).

Instead, we really ought to have a generic device for all manner of raster images (aquabsd.alps.svg can stay as it is, because we use different routines to interact with and manipulate vector images), which automatically detects formats and just opaquely returns width/height/BPP/pixels/whatever.