Make sure that the WM_opengl_context_create is always called on the main thread

Avoid the error 170 ("The requested resource is in use").

source/blender/draw/intern/draw_manager.c

@@ -1952,7 +1952,6 @@ void DRW_engines_free(void)
 void DRW_opengl_context_create(void)
 {
 	BLI_assert(DST.ogl_context == NULL); /* Ensure it's called once */
-	BLI_assert(BLI_thread_is_main());
 
 	BLI_mutex_init(&DST.ogl_context_mutex);
 

source/blender/draw/intern/draw_manager_shader.c

@@ -200,7 +200,6 @@ static void drw_deferred_shader_add(
 	BLI_addtail(&comp->queue, dsh);
 
 	/* Create one context per task. */
-	BLI_assert(BLI_thread_is_main());
 	comp->ogl_context = WM_opengl_context_create();
 	WM_opengl_context_activate(DST.ogl_context);
 

source/blender/windowmanager/intern/wm_window.c

@@ -2075,6 +2075,11 @@ void wm_window_IME_end(wmWindow *win)
 
 void *WM_opengl_context_create(void)
 {
+	/* On Windows there is a problem creating contexts that share lists
+	 * from one context that is current in another thread.
+	 * So we should call this function only on the main thread.
+	 */
+	BLI_assert(BLI_thread_is_main());
 	return GHOST_CreateOpenGLContext(g_system);
 }