Problem: getregionpos() wrong with blockwise mode and multibyte.
Solution: Use textcol and textlen instead of start_vcol and end_vcol.
Handle coladd properly (zeertzjq).
Also remove unnecessary buflist_findnr() in add_regionpos_range(), as
getregionpos() has already switched buffer.
closes: vim/vim#14805c95e64f41f