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)
 		{

src/testdir/dumps/Test_textprop_tab.dump

@@ -0,0 +1,6 @@
+| +0&#ffffff0@6> |x+0&#ffff4012@2| +0&#ffffff0@63
+|x| @6|x+0&#ffff4012@2| +0&#ffffff0@63
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|1|,|1|-|8| @8|A|l@1| 

src/testdir/test_textprop.vim

@@ -759,6 +759,26 @@ func Test_textprop_screenshot_visual()
   call RunTestVisualBlock(4, '02')
 endfunc
 
+func Test_textprop_after_tab()
+  let lines =<< trim END
+       call setline(1, [
+             \ "\txxx",
+             \ "x\txxx",
+             \ ])
+       hi SearchProp ctermbg=yellow
+       call prop_type_add('search', {'highlight': 'SearchProp'})
+       call prop_add(1, 2, {'length': 3, 'type': 'search'})
+       call prop_add(2, 3, {'length': 3, 'type': 'search'})
+  END
+  call writefile(lines, 'XtestPropTab')
+  let buf = RunVimInTerminal('-S XtestPropTab', {'rows': 6})
+  call VerifyScreenDump(buf, 'Test_textprop_tab', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XtestPropTab')
+endfunc
+
 " Adding a text property to a new buffer should not fail
 func Test_textprop_empty_buffer()
   call prop_type_add('comment', {'highlight': 'Search'})

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1571,
 /**/
     1570,
 /**/