[client] renderers: add on_help to renderer interface

This will be used to tell a render to display a help message.

client/renderers/OpenGL/opengl.c

@@ -461,6 +461,11 @@ void opengl_on_alert(void * opaque, const LG_MsgAlert alert, const char * messag
   ll_push(this->alerts, a);
 }
 
+void opengl_on_help(void * opaque, const char * message)
+{
+  // TODO: Implement this.
+}
+
 void bitmap_to_texture(LG_FontBitmap * bitmap, GLuint texture)
 {
   glBindTexture(GL_TEXTURE_2D       , texture      );
@@ -823,6 +828,7 @@ const LG_Renderer LGR_OpenGL =
   .on_frame_format = opengl_on_frame_format,
   .on_frame        = opengl_on_frame,
   .on_alert        = opengl_on_alert,
+  .on_help         = opengl_on_help,
   .render_startup  = opengl_render_startup,
   .render          = opengl_render,
   .update_fps      = opengl_update_fps