Removed implicit glfwMakeCurrentContext.

Implicitly making the context current makes sense in a
single-window API but less sense in a multi-window one.

tests/gamma.c

@@ -141,7 +141,9 @@ int main(int argc, char** argv)
 
     set_gamma(1.f);
 
+    glfwMakeContextCurrent(window);
     glfwSwapInterval(1);
+
     glfwSetKeyCallback(key_callback);
     glfwSetWindowSizeCallback(size_callback);