patch 8.1.1416: popup_getposition() not implemented yet

Problem:    Popup_getposition() not implemented yet.
Solution:   Implement it. (Yasuhiro Matsumoto, closes #4449)

runtime/doc/popup.txt

@@ -84,11 +84,17 @@ Probably 2. is the best choice.
 
 IMPLEMENTATION:
 - Code is in popupwin.c
-- implement popup_getposition({id}), use in tests
+- when creating the window set options to Vim default? (verify with 'number')
+- Do not show tilde below last line.
 - Implement filter.
+  Check that popup_close() works in the filter.
 - Handle screen resize in screenalloc().
 - Make redrawing more efficient and avoid flicker.
+    Fix redrawing problem with completion.
+    Fix redrawing problem when scrolling non-current window
+    Fix redrawing the statusline on top of a popup
 - Properly figure out the size and position.
+- Can the buffer be re-used, to avoid using up lots of buffer numbers?
 - Implement all the unimplemented options and features.
 
 
@@ -225,13 +231,16 @@ popup_getoptions({id})					*popup_getoptions()*
 		Return the {options} for popup {id}.
 
 popup_getposition({id})					*popup_getposition()*
-	  	{not implemented yet}
 		Return the position and size of popup {id}.  Returns a Dict
 		with these entries:
 			col	screen column of the popup, one-based
 			line	screen line of the popup, one-based
 			width	width of the popup in screen cells
 			height	height of the popup in screen cells
+		Note that these are the actual screen positions.  They differ
+		from the values in `popup_getoptions()` for the sizing and
+		positioning mechanism applied.
+		If popup window {id} is not found an empty Dict is returned.
 
 win_execute({id}, {command})
 	  	{not implemented yet}