add T key for toggling the image fill mode

[artiface]

First thanks for this tool! I have been looking for a simple slide show like this for a while, and the random/sequential toggles are exactly what is missing from so many.

In the original the images were always using UniformToFill mode, which would stretch them off the screen depending on original size. I added a T key to toggle the fill mode between

• Uniform: fits the whole image, letterboxed
• UniformToFill: fills the screen, cropping the edges (the original)
• Fill: stretches to fill exactly, distorting aspect ratio
• None: displays at original pixel size, no scaling

Also added the StretchMode to the config file, defaulted to Uniform.

Also fixed minor bug in where the Ken Burns pan (maxPan = 40 pixels) can push a Uniform-stretched image slightly off-centre and reveal black edges. Now it's calculated as a fraction of the extra space the zoom creates, so the pan is always safely within the zoomed bounds and won't slide the image off-screen.