Changed caret preferences: The default image capture type is 'render pixmap'...

Changed caret preferences: The default image capture type is 'render pixmap' which uses an offscreen pixmap for creating the image.  However, Linux systems have problems with the 'render pixmap' so the default for Linux has been changed to 'grab framebuffer'.  This change will only affect new users who have not run Workbench and thus do not yet have a preferences file.