Change: make GetPartialZ consistent, meaning Z of adjacent slopes continue

Previously, on a straight line of a one corner up slope with the adjacent
steep sloop the Z would increase one step every two sub pixels, except for one
case where one sub pixel is skipped. Similarly, a steep slope with two
adjacent one corner up slopes, would have a bump in the height line along the
diagonal whenever it enters/leaves the steep slope tile.

src/saveload/saveload.h

@@ -348,6 +348,7 @@ enum SaveLoadVersion : uint16 {
 	SLV_LINKGRAPH_EDGES,                    ///< 304  PR#10314 Explicitly store link graph edges destination, PR#10471 int64 instead of uint64 league rating
 
 	SLV_VELOCITY_NAUTICAL,                  ///< 305  PR#10594 Separation of land and nautical velocity (knots!)
+	SLV_CONSISTENT_PARTIAL_Z,               ///< 306  PR#10570 Conversion from an inconsistent partial Z calculation for slopes, to one that is (more) consistent.
 
 	SL_MAX_VERSION,                         ///< Highest possible saveload version
 };