Add path auto-tiling tool to map editor

Exposes the TilingPath code used by the map generator as a map editor
tool to help automate the process of tiling sequences of templates
together.

No additional template stitching definitions are added as part of this
commit. As such, the tool currently only supports the same auto tiling
as the map generator does. This includes:

- RA and CnC support.
- Beaches, land cliffs, and roads.

Support for additional mods and templates can be added in follow-ups
by adding the necessary MultiBrush definitions.

Related changes included in this commit to support this work include:

- Add support for sparse EditorBlits.
- Add support for MultiBrush to EditorBlitSource conversion.
- Add support for EditorBlitSource previewing.
- Adjust the "Road" types used in segmented MultiBrush definitions.

OpenRA.Mods.Common/Traits/World/EditorActorPreview.cs

@@ -137,7 +137,17 @@ namespace OpenRA.Mods.Common.Traits
 
 		public IEnumerable<IRenderable> Render()
 		{
-			var items = previews.SelectMany(p => p.Render(worldRenderer, CenterPosition));
+			return RenderAt(CenterPosition);
+		}
+
+		public IEnumerable<IRenderable> RenderWithOffset(WVec offset)
+		{
+			return RenderAt(CenterPosition + offset);
+		}
+
+		public IEnumerable<IRenderable> RenderAt(WPos centerPosition)
+		{
+			var items = previews.SelectMany(p => p.Render(worldRenderer, centerPosition));
 			if (Selected)
 			{
 				var overlay = items.Where(r => !r.IsDecoration && r is IModifyableRenderable)