Returns the byte offset of a line (0-indexed). |api-indexing| Line 1 (index=0) has offset 0. UTF-8 bytes are counted. EOL is one byte. 'fileformat' and 'fileencoding' are ignored. The line index just after the last line gives the total byte-count of the buffer. A final EOL byte is counted if it would be written, see 'eol'. Unlike |line2byte()|, throws error for out-of-bounds indexing. Returns -1 for unloaded buffer. Parameters: ~ {buffer} Buffer handle, or 0 for current buffer {index} Line index Return: ~ Integer byte offset, or -1 for unloaded buffer.