patch 8.1.1571: textprop highlight starts too early if just after a tab

Problem:    textprop highlight starts too early if just after a tab.
Solution:   Check if still drawing a previous character. (closes #4558)

src/screen.c

@@ -4391,6 +4391,9 @@ win_line(
 		int pi;
 		int bcol = (int)(ptr - line);
 
+		if (n_extra > 0)
+		    --bcol;  // still working on the previous char, e.g. Tab
+
 		// Check if any active property ends.
 		for (pi = 0; pi < text_props_active; ++pi)
 		{