Wayland: Fix partial writes of clipboard string

The string pointer used to write the contents of our clipboard data
offer was never updated, causing it to repeat parts of the beginning of
the string until the correct number of bytes had been written.

src/wl_window.c

@@ -1591,7 +1591,7 @@ static void dataSourceHandleSend(void* data,
                                  const char* mimeType,
                                  int fd)
 {
-    const char* string = _glfw.wl.clipboardSendString;
+    char* string = _glfw.wl.clipboardSendString;
     size_t len = strlen(string);
     int ret;
 
@@ -1631,6 +1631,7 @@ static void dataSourceHandleSend(void* data,
             return;
         }
         len -= ret;
+        string += ret;
     }
     close(fd);
 }