fix(winfloat): last_status when changing split to floatwin

Problem: converting a split to a floatwin may not remove the last statusline
when needed. (e.g: 'ls' is 1)

Solution: call last_status/win_comp_pos in win_new_float, after win_remove.

Also fix float_pos formatting for screen snapshots so it doesn't give a nil
error for external windows.

Not an issue from this PR.

test/functional/ui/screen.lua

@@ -1795,9 +1795,14 @@ local function fmt_ext_state(name, state)
   elseif name == 'float_pos' then
     local str = '{\n'
     for k, v in pairs(state) do
-      str = str .. '  [' .. k .. '] = {' .. v[1]
-      for i = 2, #v do
-        str = str .. ', ' .. inspect(v[i])
+      str = str .. '  [' .. k .. '] = {'
+      if v.external then
+        str = str .. ' external = true '
+      else
+        str = str .. v[1]
+        for i = 2, #v do
+          str = str .. ', ' .. inspect(v[i])
+        end
       end
       str = str .. '};\n'
     end