Support for autotile (RPG Maker format)

Tiled is a great tool, especially for those who use Unity or game engines other than RPG maker. With the release of the newest humble bundle I foresee many people like myself wanting to use their new assets with Tiled (and eventually game engines other than RPG maker). It appears that their Autotile format is quite common and works well for compacting things, but makes it difficult to use in Tiled without an external tool.

I think it is within the scope of Tiled to support importing this format and then automatically slicing it up. While the terrain tool works well for some sprites, it doesn't seem to work with this format. There is an existing tool Remex but it only supports the 64x96 pixel image at a time. It would be better to automatically detect the size and split it accordingly (It appears 512x384 is standard)

Start by examining Tiled's current terrain tool and tileset image-import or slicing entry points, then compare their behavior with the Remex tool linked in the issue. Done means importing RPG Maker autotile images, detecting supported dimensions such as 64x96 or 512x384, and splitting them into usable tilesets without an external tool.