Fixes loading external Tiled tilesets with relative images

[xSke]

This commit fixes a bug when loading external tilesets with external images.

Consider the following directory structure:

root/
    tilemap.tmx [refers to tileset as "tiles/tileset.tsx"]
    tiles/
        tileset.tsx [refers to images as "tile#.png"]
        tile1.png
        tile2.png

Previously, it would try to load the images using the tilemap's location as a base, so it would look for images in root/tile#.png. Now, it will load from root/tiles/tile#.png as expected.