Tweak the mechanics for showing and rendering lua screens.

- Place new screens below any dismissed ones on top.
- When asked to render a dismissed lua screen, call render() on its
  parent to avoid producing a black screen frame when unlucky.

library/lua/gui.lua

@@ -513,17 +513,18 @@ function Screen:sendInputToParent(...)
     end
 end
 
-function Screen:show(below)
+function Screen:show(parent)
     if self._native then
         error("This screen is already on display")
     end
-    self:onAboutToShow(below)
-    if not dscreen.show(self, below) then
+    parent = parent or dfhack.gui.getCurViewscreen(true)
+    self:onAboutToShow(parent)
+    if not dscreen.show(self, parent.child) then
         error('Could not show screen')
     end
 end
 
-function Screen:onAboutToShow()
+function Screen:onAboutToShow(parent)
 end
 
 function Screen:onShow()

library/lua/gui/dwarfmode.lua

@@ -373,10 +373,8 @@ function DwarfOverlay:simulateCursorMovement(keys, anchor)
     end
 end
 
-function DwarfOverlay:onAboutToShow(below)
-    local screen = dfhack.gui.getCurViewscreen()
-    if below then screen = below.parent end
-    if not df.viewscreen_dwarfmodest:is_instance(screen) then
+function DwarfOverlay:onAboutToShow(parent)
+    if not df.viewscreen_dwarfmodest:is_instance(parent) then
         error("This screen requires the main dwarfmode view")
     end
 end

library/modules/Screen.cpp

@@ -639,7 +639,12 @@ dfhack_lua_viewscreen::~dfhack_lua_viewscreen()
 
 void dfhack_lua_viewscreen::render()
 {
-    if (Screen::isDismissed(this)) return;
+    if (Screen::isDismissed(this))
+    {
+        if (parent)
+            parent->render();
+        return;
+    }
 
     dfhack_viewscreen::render();