patch 8.1.1050: blank srceen when DirectWrite failed

Problem:    Blank srceen when DirectWrite failed.
Solution:   Call redraw_later_clear() after recreating the Direct2D render
            target. (Ken Takata, closes #4172)
diff --git a/src/gui_dwrite.cpp b/src/gui_dwrite.cpp
index 4a24a03..3c477d0 100644
--- a/src/gui_dwrite.cpp
+++ b/src/gui_dwrite.cpp
@@ -930,6 +930,8 @@
     }
 }
 
+extern "C" void redraw_later_clear(void);
+
     HRESULT
 DWriteContext::SetDrawingMode(DrawingMode mode)
 {
@@ -952,6 +954,7 @@
 		    hr = S_OK;
 		    DiscardDeviceResources();
 		    CreateDeviceResources();
+		    redraw_later_clear();
 		}
 		mDrawing = false;
 	    }