Add service for extraction dimensions from SVG image

Here is a patch.

@Berdir suggested that we return FALSE if width or height is not defined, adjusted patch for that.

Tested this a bit against various projects logo examples.
Works great but there is a problem, most of the time logo has width and height wihout unit, sometimes in px and sometimes in mm.
Checked SVG specification against it and units are allowed for SVG width and height. But for image element you can specify only values without units.

I guess the service needs to remove units from width and height values when they exist.

Here is a new version which removes units.

Committed.

Automatically closed - issue fixed for 2 weeks with no activity.