gstreamer/buffer: Unmap memory in flush() of the Write impl of the buffer cursor

This makes sure that any remaining data is actually written out as part
of the unmapping, which might be necessary depending on the memory type.

gstreamer/src/buffer_cursor.rs

@@ -218,6 +218,13 @@ macro_rules! define_write_impl(
         }
 
         fn flush(&mut self) -> Result<(), io::Error> {
+            if !self.map_info.memory.is_null() {
+                unsafe {
+                    gst_sys::gst_memory_unmap(self.map_info.memory, &mut self.map_info);
+                    self.map_info.memory = ptr::null_mut();
+                }
+            }
+
             Ok(())
         }
     }