Initializes @iter to a position @char_offset chars from the start
of the entire buffer.
If @char_offset is -1 or greater than the number
of characters in the buffer, @iter is initialized to the end iterator,
the iterator one past the last valid character in the buffer.
Initializes @iter to a position @char_offset chars from the start of the entire buffer.
If @char_offset is -1 or greater than the number of characters in the buffer, @iter is initialized to the end iterator, the iterator one past the last valid character in the buffer.