patch 9.1.0056: wrong number of trailing spaces inserted after blockwise put

patch 9.1.0056: wrong number of trailing spaces inserted after blockwise put

Problem:  Incorrect number of trailing spaces inserted for multibyte
	  characters when pasting a blockwise register in blockwise visual
          mode (VanaIgr)
Solution: Skip over trailing UTF-8 bytes when computing the number of trailing
          spaces (VanaIgr)

When pasting in blockwise visual mode, and the register type is <CTRL-V>, Vim
aligns the text after the replaced area by inserting spaces after pasted
lines that are shorter than the longest line. When a shorter line contains
multibyte characters, each trailing UTF-8 byte's width is counted in addition
to the width of the character itself. Each trailing byte counts as being 4
cells wide (since it would be displayed as <xx>).

closes: #13909

Signed-off-by: VanaIgr <vanaigranov@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>