_lv_txt_get_next_line() function
Get the next line of text. Check line length and break chars too.
Arguments
txt
a '\0' terminated string
max_width
max width of the text (break the lines to fit this size). Set COORD_MAX to avoid line breaks
used_width
When used_width != NULL, save the width of this line if flag == LV_TEXT_FLAG_NONE, otherwise save -1.
Return value
the index of the first char of the new line (in byte index not letter index. With UTF-8 they are different)