vim-patch:8.1.0307: there is no good way to get the window layout

Problem:    There is no good way to get the window layout.
Solution:   Add the winlayout() function. (Yegappan Lakshmanan)
0f6b4f06de

src/nvim/eval.lua

@@ -352,6 +352,7 @@ return {
     winbufnr={args=1},
     wincol={},
     winheight={args=1},
+    winlayout={args={0, 1}},
     winline={},
     winnr={args={0, 1}},
     winrestcmd={},