wayland: surface: add function for releasing a buffer

Releasing the current buffer has no major effect because the program is
double-buffered, but add a function to release the buffer anyway.

src/wayland/surface.rs

@@ -105,6 +105,11 @@ impl<T: Dispatch<WlBuffer, u32>
         self.scale = scale;
     }
 
+    pub fn release(&mut self, id: u32)
+    {
+        self.bufs[id as usize].release();
+    }
+
     pub fn hide(&mut self)
     {
         self.configured = false;