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/tests/sharing.c b/tests/sharing.c
index 8031a9b..95f2134 100644
--- a/tests/sharing.c
+++ b/tests/sharing.c
@@ -68,9 +68,11 @@
     if (!window)
         return NULL;
 
+    glfwMakeContextCurrent(window);
+    glfwSwapInterval(1);
+
     glfwSetWindowCloseCallback(window_close_callback);
     glfwSetKeyCallback(key_callback);
-    glfwSwapInterval(1);
 
     return window;
 }