Add "Fill and Crop" option for wf-background. Pt 2

[trigg]

I felt strongly about #58 so I replicated the fork against master and followed the requested changes.

After some testing with a variety of images and screens I made further edits:

• Added an image_scale value that gets passed into the render logic for each image
• Added cairo context save/restore and placed a scale inside it to allow the image to be scaled up in-place without loading it a second time.

These were necessary as loading tall images in ultrawide screens or visa-versa caused a crash from attempting to create a Pixbuf that was too large.

I have attempted to stick to the coding style of the project but feel free to correct me on it.

[ammen99]

Tested and seems to work, thanks!