Introduce initial PPos plumbing.

PPos is best thought of as a cell grid applied in
screen space.  Multiple cells with different
terrain heights may be projected to the same PPos,
or to multiple PPos if they do not align with the
screen grid.

PPos coordinates are used primarily for map edge
checks and shroud / visibility queries.

OpenRA.Game/Graphics/TerrainSpriteLayer.cs

@@ -98,8 +98,8 @@ namespace OpenRA.Graphics
 			var cells = restrictToBounds ? viewport.VisibleCellsInsideBounds : viewport.AllVisibleCells;
 
 			// Only draw the rows that are visible.
-			var firstRow = cells.MapCoords.TopLeft.V;
-			var lastRow = Math.Min(cells.MapCoords.BottomRight.V + 1, map.MapSize.Y);
+			var firstRow = cells.CandidateMapCoords.TopLeft.V.Clamp(0, map.MapSize.Y);
+			var lastRow = (cells.CandidateMapCoords.BottomRight.V + 1).Clamp(firstRow, map.MapSize.Y);
 
 			Game.Renderer.Flush();