Fix for artifacts on blended wallpaper and support for "Fill" style.

I found this program very usefull, but I personally needed the following changes.
On a bright screen with a dark wallpaper, the blend seemed to leave artifacts on the final image. Now it replaces the final blended image with the original new image (without any blend) after doing the transition. 
I also added support for "Fill" style, which is usefull for users with multiple monitors of different proportions (like me).

Excuse my not-too-good use of git, still learning.