Provide full QuickRefresh interface for 4in2 too

This display needs no special commands for displaying the new frame. But
it will come in handy to support the full QuickRefresh trait when it
comes to supporting different quick refresh capable displays from an
application.

src/epd4in2/mod.rs

@@ -482,21 +482,24 @@ where
         Ok(())
     }
 
-    /// This function is not needed for this display
-    #[allow(unused)]
+    /// This is wrapper around `display_frame` for using this device as a true
+    /// `QuickRefresh` device.
     fn display_new_frame(&mut self, spi: &mut SPI, delay: &mut DELAY) -> Result<(), SPI::Error> {
-        unimplemented!()
+        self.display_frame(spi, delay)
     }
 
-    /// This function is not needed for this display
-    #[allow(unused)]
+    /// This is wrapper around `update_new_frame` and `display_frame` for using
+    /// this device as a true `QuickRefresh` device.
+    ///
+    /// To be used immediately after `update_old_frame`.
     fn update_and_display_new_frame(
         &mut self,
         spi: &mut SPI,
         buffer: &[u8],
         delay: &mut DELAY,
     ) -> Result<(), SPI::Error> {
-        unimplemented!()
+        self.update_new_frame(spi, buffer, delay)?;
+        self.display_frame(spi, delay)
     }
 
     fn update_partial_old_frame(