Images: handle rowstride != width*bytes_per_pixel

Up till now, image data with a rowstride larger than the value
expected from width and bytes_per_pixel caused a trapezoid
distortion of the displayed image.

Signed-off-by: Olaf Mandel <o.mandel@menlosystems.com>
Make appearance more configurable

 * Allow not showing the startup message
 * Make the screen-split (between image and progress bar)
   configurable
 * Allow for fullscreen image (overlaid by progress bar)

Signed-off-by: Olaf Mandel <o.mandel@menlosystems.com>