GL: Wrap extension support inside GLContext

This makes it possible to disable all the extensions when forcing
workarounds.

Also it will allow future options to selectively disable each extension
to know which one is buggy.

source/blender/gpu/opengl/gl_vertex_array.cc

@@ -135,7 +135,7 @@ void GLVertArray::update_bindings(const GLuint vao,
     }
   }
 
-  if (attr_mask != 0 && GLEW_ARB_vertex_attrib_binding) {
+  if (attr_mask != 0 && GLContext::vertex_attrib_binding_support) {
     for (uint16_t mask = 1, a = 0; a < 16; a++, mask <<= 1) {
       if (attr_mask & mask) {
         GLContext *ctx = GLContext::get();