If the viewing area has been separated into several layers, the
application may need to find out which layer is topmost at a particular
x,y coordinate.  Use the whichlayer() function for this:

    struct Layer *WhichLayer( struct Layer_Info *li, long x, long y );

To be sure that no task adds, deletes, or changes the sequence of layers
before this information is used, call locklayerinfo() before calling
whichlayer(), and call unlocklayerinfo() when the operation is complete.
In this way, the program may ensure that it is acting on valid
information.  Always check for a NULL return value (coordinate not in a
layer) from WhichLayer().