X11: Add additional XKB key names for Right Alt

(cherry picked from commit 318e08d914)

src/x11_init.c

@@ -336,6 +336,8 @@ static void createKeyTables(void)
             { GLFW_KEY_RIGHT_SHIFT, "RTSH" },
             { GLFW_KEY_RIGHT_CONTROL, "RCTL" },
             { GLFW_KEY_RIGHT_ALT, "RALT" },
+            { GLFW_KEY_RIGHT_ALT, "LVL3" },
+            { GLFW_KEY_RIGHT_ALT, "MDSW" },
             { GLFW_KEY_RIGHT_SUPER, "RWIN" },
             { GLFW_KEY_MENU, "MENU" }
         };