Smarter behavior when selecting multiple tiles from the tileset

[bjorn]

Quoted from @eevee's feedback:

The ability to draw with a selection of several tiles is great! Alas! If I select a block of off-sized tiles from a tileset, Tiled will try to draw them in adjacent map tiles, rather than spacing them out to preserve their appearance as a block.

I know this is unsolvable in the general case, but I've only ever wanted to do it with a tileset whose size is an integral multiple of the map's tile size.

It would be possible to detect this case and be a little smarter when building up the tile stamp in TilesetDock::updateCurrentTiles.

Care should be taken that the behavior doesn't confuse the user though, and it only makes sense for orthogonal maps.