Handle tileset margins and spacing

[bcvery1]

TileSets can have margins and tile spacing. This is currently unhandled, so tile sprites will be incorrect if either of these values are set.

The calculation is done in utilities.go, but the calling code is in tile.go in the function Draw which already takes a *Tileset parameter, shouldn't be difficult to calculate the offsets

[perber]

Hello, I`m actually using your library and I would like to create a PR which adresses this issue. My map is currently using a 1pixel spacing.

[bcvery1]

Hi @perber - sorry about getting back to this so late! That's great to hear that TilePix is being used.

Hopefully we can get your PR merged soon so you can make use of the spacing on your tileset