Replace glad and the Vulkan SDK with glad2

This removes all dependencies from the GLFW test programs on the Vulkan
SDK.

It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.

examples/sharing.c

@@ -23,7 +23,7 @@
 //
 //========================================================================
 
-#include <glad/glad.h>
+#include <glad/gl.h>
 #include <GLFW/glfw3.h>
 
 #include <stdio.h>
@@ -103,7 +103,7 @@ int main(int argc, char** argv)
 
     // The contexts are created with the same APIs so the function
     // pointers should be re-usable between them
-    gladLoadGLLoader((GLADloadproc) glfwGetProcAddress);
+    gladLoadGL(glfwGetProcAddress);
 
     // Create the OpenGL objects inside the first context, created above
     // All objects will be shared with the second context, created below