color picker: backtransform picker coordinates; fixes #9482

coordinates of local color picker need to be transformed back from the preview
image to the module in focus, else we pick completely bogus spots if distorting
modules are in action.
color picker: better account for invalid local color picker data in several places

if the local color picker is positioned outside the module's image area it can not
deliver valid data. take appropriate action in this case (depends on where we use
the color picker).