Rivers - Add Delta Support

[QuintillusCFC]

The file deltaRivers.pcx contains four delta graphics. These are displayed when rivers empty into the coast (exact formula TBD). We should include them in our river graphics.

[QuintillusCFC]

Basic Java code for this is located in my editor repo at https://hg.sr.ht/~adj/civ3_cross_platform_editor/browse/src/main/java/com/civfanatics/civ3/xplatformeditor/tabs/map/renderer/ClassicRenderer.java?rev=2aefc36c13bac3b863aed1920c54381243d0cdc4#L857 . There is also a diff for the change set at https://hg.sr.ht/~adj/civ3_cross_platform_editor/rev/2aefc36c13bac3b863aed1920c54381243d0cdc4, if you prefer that format.

Note that this version requires two neighboring coastal tiles; I believe in some (all?) cases, one may be sufficient, but additional testing (in the Firaxis editor) should be done to see whether one coastal tile is always sufficient for delta graphics or not.

[QuintillusCFC]

Observing a random Civ map, it appears that all rivers end next to two coast tiles. Not sure if that's a 100% true statement, but it was on two maps (one Huge) that I observed.

In scenarios where that isn't true, it appears to be inconsistent whether a delta is included where a river ends.

For now I propose we display deltas when there are two neighboring coastal tiles.

[benskywalker]

PR #431 is working on this on the tilemaps branch. There are still some alignment issues, but deltas show up now