Move graphics-relevant stuff into extra unit

[frostney]

Probably some interfaces or something. Not completely sure how to exactly implement it.

I think the prettiest way would be extend TelNode.Draw to something like that:

TelNode.Draw(Graphics: TelGraphicsProvider);
begin
Graphics.DrawLine(...);
Graphics.DrawTexture(...);
end;

The other way would be to avoid circular references as much possible, which could turn out to be difficult. (Like uses graphics-related units only in the implementation section, class helpers or similar.)