Resize to size of background image

[makoehr]

This changes the behaviour of dalliclick.sh in the following way:

• If a background image is specified, the output files will have exactly the size as the background image. The input image will be scaled before masking it. If the ratio of the input file does not fit to the background image it will be filled with a color. This color is selectable via the new option -B

This enhancement eases the processing of multiple input files that have different sizes.