Make tests and examples use glad

tests/gamma.c

@@ -28,6 +28,7 @@
 //
 //========================================================================
 
+#include <glad/glad.h>
 #include <GLFW/glfw3.h>
 
 #include <stdio.h>
@@ -153,6 +154,7 @@ int main(int argc, char** argv)
     set_gamma(window, 1.f);
 
     glfwMakeContextCurrent(window);
+    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
     glfwSwapInterval(1);
 
     glfwSetKeyCallback(window, key_callback);