only do a full refresh when needed

this significantly reduces CPU utilization when DFHack-owned screens are
visible.

docs/dev/Lua API.rst

@@ -2363,7 +2363,14 @@ Supported callbacks and fields are:
   where the above painting functions work correctly.
 
   If omitted, the screen is cleared; otherwise it should do that itself.
-  In order to make a see-through dialog, call ``self._native.parent:render()``.
+  In order to make a dialog where portions of the parent viewscreen are still
+  visible in the background, call ``screen:renderParent()``.
+
+  If artifacts are left on the parent even after this function is called, such
+  as when the window is dragged or is resized, any code can set
+  ``gui.Screen.request_full_screen_refresh`` to ``true``. Then when
+  ``screen.renderParent()`` is next called, it will do a full flush of the
+  graphics and clear the screen of artifacts.
 
 * ``function screen:onIdle()``
 

library/lua/gui.lua

@@ -597,6 +597,7 @@ end
 Screen = defclass(Screen, View)
 
 Screen.text_input_mode = false
+Screen.request_full_screen_refresh = false
 
 function Screen:postinit()
     self:onResize(dscreen.getWindowSize())
@@ -622,7 +623,10 @@ function Screen:renderParent()
     else
         dscreen.clear()
     end
-    df.global.gps.force_full_display_count = 1
+    if Screen.request_full_screen_refresh then
+        df.global.gps.force_full_display_count = 1
+        Screen.request_full_screen_refresh = false
+    end
 end
 
 function Screen:sendInputToParent(...)

library/lua/gui/widgets.lua

@@ -487,6 +487,10 @@ Window.ATTRS {
     draggable = true,
 }
 
+function Window:postUpdateLayout()
+    gui.Screen.request_full_screen_refresh = true
+end
+
 -------------------
 -- ResizingPanel --
 -------------------