Add river delta textures

[ajhalme]

Fixes #107, missing river delta textures. See also Draft PR #433 about a tile map variant, linked to #407.

There are two river textures in the Civ3 Art assets, a regular set and one for river deltas, to be drawn when the river meets the sea. The delta textures are not currently being loaded in.

This patch adds the textures, and applies according to a simple heuristic. Given the close relationship between the two tile sets and how they are deployed, the implementation reduces to a simple check to see whether the tile drawing is happening next to water tiles or not.

Sample save file

Given how closely related to the regular river tiles the delta tiles are, regardless of how the tile map effort proceeds, #433 can probably be closed off.

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Note the non-delta river segments in the delta texture:

[stavrosfa]

Thanks! I tested it with a few maps, looks good to me.

[stavrosfa]

Hmm, I think I found something.

Editor

vs

OpenCiv3

I think we need to also check that it's the "ending" of the river too, so only one neighbor can have a river if it's a delta