Change mxn.util.getSubdomainTileURL to return predictable subdomain

[gilesc50]

mxn.util.getSubdomainTileURL currently returns a random subdomain, but if there are 4 subdomains then make it use a formula taking in the X and Y coordintates:

X odd, Y odd: Subdomain 0
X even, Y odd: Subdomain 1
X odd, Y even: Subdomain 2
X even, Y even: Subdomain 3

so that it still spreads the load, but that once a tile is cached it always come from the same url so is not rerequested.