"Blank" tile ignoring

[maxim-zhao]

If I have multiple tilesets to be used at the same time, I may want to have a "blank" tile that they all share, so I can use arbitrary layouts. Adding a blank row at the top of the images can ensure that it is the first one. An option to discard this tile would allow the tiles to avoid duplicating it; an option to substitute it with another index would allow the tilemap to be correct for any blank tiles encountered.