vim-patch:8.2.4559: getmousepos() returns the screen column

Problem:    getmousepos() returns the screen column. (Ernie Rael)
Solution:   Return the text column, as documented.
533870a985

Re-introduce vcol2col, which was removed in 71b1f4e for being unused.
Move it to mouse.c (like in v8.1.2062, which hasn't been ported yet).

src/nvim/testdir/test_functions.vim

@@ -1765,6 +1765,18 @@ endfunc
 func Test_getmousepos()
   enew!
   call setline(1, "\t\t\t1234")
+  " call test_setmouse(1, 1)
+  call nvim_input_mouse('left', 'press', '', 0, 0, 0)
+  call getchar() " wait for and consume the mouse press
+  call assert_equal(#{
+        \ screenrow: 1,
+        \ screencol: 1,
+        \ winid: win_getid(),
+        \ winrow: 1,
+        \ wincol: 1,
+        \ line: 1,
+        \ column: 1,
+        \ }, getmousepos())
   " call test_setmouse(1, 25)
   call nvim_input_mouse('left', 'press', '', 0, 0, 24)
   call getchar() " wait for and consume the mouse press
@@ -1775,7 +1787,7 @@ func Test_getmousepos()
         \ winrow: 1,
         \ wincol: 25,
         \ line: 1,
-        \ column: 25,
+        \ column: 4,
         \ }, getmousepos())
   " call test_setmouse(1, 50)
   call nvim_input_mouse('left', 'press', '', 0, 0, 49)
@@ -1787,7 +1799,7 @@ func Test_getmousepos()
         \ winrow: 1,
         \ wincol: 50,
         \ line: 1,
-        \ column: 29,
+        \ column: 8,
         \ }, getmousepos())
   bwipe!
 endfunc