Patch to add support for saving screen shot as jpeg file

Submitted by Gert Wollny

Assigned to Jérôme Guelfucci

Hello,

the attached patch add the option -j/--jpeg to enable saving as jpeg file.

With this option given:

the image will be saved to jpeg file with the default jpeg options used in the gdk_save_image function.
in the file dialog the file name is presented with the 'jpg' extension. If this extension is changed to anything else, a png file will be saved.

Independently. if in the file dialog the extension is set to 'jpg', the image is also saved as jpeg file.

best regards, Gert