Removed implicit glfwMakeCurrentContext.

Implicitly making the context current makes sense in a
single-window API but less sense in a multi-window one.
diff --git a/examples/boing.c b/examples/boing.c
index c0d3f0e..3cc0973 100644
--- a/examples/boing.c
+++ b/examples/boing.c
@@ -589,11 +589,13 @@
        exit( EXIT_FAILURE );
    }
 
+   glfwMakeContextCurrent(window);
+   glfwSwapInterval( 1 );
+
    glfwGetWindowSize(window, &width, &height);
    reshape(window, width, height);
 
    glfwSetInputMode( window, GLFW_STICKY_KEYS, GL_TRUE );
-   glfwSwapInterval( 1 );
    glfwSetTime( 0.0 );
 
    init();