ScreenshotSelectionWindow: assume the user aborted if the selection_width or selection_height are 0.
ScreenshotSelectionWindow: handle cases when end < start
ScreenshotSelectionWindow: display the black mask even when the selection has not been started.
ScreenshotSelectionWindow: use vera-color if available for the mask color.
ScreenshotSelectionWindow: bugfix for selection_source.
Added screenshot selection support in vera-command.