glthread: add GL_DRAW_INDIRECT_BUFFER tracking and generator support

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3948>

src/mesa/main/glthread.h

@@ -94,6 +94,7 @@ struct glthread_state
     * buffer) binding is in a VBO.
     */
    bool element_array_is_vbo;
+   bool draw_indirect_buffer_is_vbo;
 };
 
 void _mesa_glthread_init(struct gl_context *ctx);