Allow specifying render width/height of tile draw layers

[Ruin0x11]

For the screenshot feature to work, we'd have to be able to ask the renderer to output an image at the same size as the entire map the player is in. But currently there is no width or height parameter to determine the width and height of the visible area in IDrawLayer:draw(x, y). We should force this method to accept width and height also, without having to use ICoords:get_start_offset().