patch 8.0.1369: MS-Windows: drawing underline slow, mFallbackDC not updated

Problem:    MS-Windows: drawing underline, curl and strike-throw is slow,
            mFallbackDC not properly updated.
Solution:   Several performance improvements. (Ken Takata, Taro Muraoka,
            Yasuhiro Matsumoto, closes #2401)
diff --git a/src/gui_dwrite.h b/src/gui_dwrite.h
index 8ab2864..9c98c4c 100644
--- a/src/gui_dwrite.h
+++ b/src/gui_dwrite.h
@@ -55,11 +55,11 @@
 void DWrite_Final(void);
 
 DWriteContext *DWriteContext_Open(void);
-void DWriteContext_BindDC(DWriteContext *ctx, HDC hdc, RECT *rect);
+void DWriteContext_BindDC(DWriteContext *ctx, HDC hdc, const RECT *rect);
 void DWriteContext_SetFont(DWriteContext *ctx, HFONT hFont);
 void DWriteContext_DrawText(
 	DWriteContext *ctx,
-	const WCHAR* text,
+	const WCHAR *text,
 	int len,
 	int x,
 	int y,
@@ -68,9 +68,12 @@
 	int cellWidth,
 	COLORREF color,
 	UINT fuOptions,
-	CONST RECT *lprc,
-	CONST INT * lpDx);
-void DWriteContext_FillRect(DWriteContext *ctx, RECT *rc, COLORREF color);
+	const RECT *lprc,
+	const INT *lpDx);
+void DWriteContext_FillRect(DWriteContext *ctx, const RECT *rc, COLORREF color);
+void DWriteContext_DrawLine(DWriteContext *ctx, int x1, int y1, int x2, int y2,
+	COLORREF color);
+void DWriteContext_SetPixel(DWriteContext *ctx, int x, int y, COLORREF color);
 void DWriteContext_Flush(DWriteContext *ctx);
 void DWriteContext_Close(DWriteContext *ctx);