Retrieves an array of logical attributes for each character in the layout.
This is a faster alternative to Layout.getLogAttrs. The returned array is part of layout and must not be modified. Modifying the layout will invalidate the returned array.
The number of attributes returned in n_attrs will be one more than the total number of characters in the layout, since there need to be attributes corresponding to both the position before the first character and the position after the last character.