forward_iter_to_source_mark
Description:
Moves `iter` to the position of the next [class@Mark] of the given `category`.
Returns true if `iter` was moved. If `category` is NULL, the next source mark can be of any category.
Parameters:
| this | a Buffer. | 
| iter | an iterator. | 
| category | category to search for, or null | 
Returns:
| whether `iter` was moved. |