First pass of MeshDescription API and format refactor.

- Removed hardcoded element type arrays (Vertices, Edges, Triangles etc.). Mesh element types can now be arbitrarily added, with any number of channels.
- Mesh element containers have a much leaner format; instead of sparse arrays, they are now represented by a simple bitarray, determining whether an index is used or not. Consequently, mesh topology is now entirely described with the attribute system, e.g. edge start and end vertices, triangle vertices, etc.
- Support added for attributes of arbitrary dimensions, e.g. float[4] or int[2].
- Support added for attributes which index into another mesh element container.
- Added FMeshElementIndexer: this is an efficient container for maintaining backward references from one element type to another; for example, edges have an attribute specifying which vertices are at each end (an attribute of type FVertexID[2]). With an indexer, it is possible to look up which edges contain a given vertex, even though this is not explicitly stored. Indexers are designed to do minimal allocations and update lazily and in batch when necessary.
- Added support for preserving UV topology in static meshes. UVs are now a first-class element type which may be indexed directly from triangles.
- Added the facility to access the underlying array in an attribute array directly.
- Triangles now directly reference their vertex, edge and UV IDs. Vertex instances are to be deprecated.
- Changed various systems to be triangle-centric rather than polygon-centric, as this is faster. Triangles are presumed to be the elementary face type in a MeshDescription, even if polygons are still supported. The concept of polygons will be somewhat shifted to mean a group of triangles which should be treated collectively for editing purposes.
- Optimised normal/tangent generation and FBX import.
- Deprecated EditableMesh, MeshEditor and StaticMeshEditorExtension plugins - these are to be removed, but they still have certain hooks in place which need removing.
#rb

[CL 13568702 by Richard TalbotWatkin in ue5-main branch]

Engine/Source/Developer/MeshBuilder/Private/MeshDescriptionHelper.cpp

@@ -40,7 +40,7 @@ void FMeshDescriptionHelper::SetupRenderMeshDescription(UObject* Owner, FMeshDes
 	float ComparisonThreshold = BuildSettings->bRemoveDegenerates ? THRESH_POINTS_ARE_SAME : 0.0f;
 	
 	//This function make sure the Polygon Normals Tangents Binormals are computed and also remove degenerated triangle from the render mesh description.
-	FStaticMeshOperations::ComputePolygonTangentsAndNormals(RenderMeshDescription, ComparisonThreshold);
+	FStaticMeshOperations::ComputeTriangleTangentsAndNormals(RenderMeshDescription, ComparisonThreshold);
 	//OutRenderMeshDescription->ComputePolygonTangentsAndNormals(BuildSettings->bRemoveDegenerates ? SMALL_NUMBER : 0.0f);
 
 	FVertexInstanceArray& VertexInstanceArray = RenderMeshDescription.VertexInstances();
@@ -67,7 +67,7 @@ void FMeshDescriptionHelper::SetupRenderMeshDescription(UObject* Owner, FMeshDes
 	if (BuildSettings->bGenerateLightmapUVs && VertexInstanceArray.Num() > 0)
 	{
 		TVertexInstanceAttributesRef<FVector2D> VertexInstanceUVs = RenderMeshDescription.VertexInstanceAttributes().GetAttributesRef<FVector2D>(MeshAttribute::VertexInstance::TextureCoordinate);
-		int32 NumIndices = VertexInstanceUVs.GetNumIndices();
+		int32 NumIndices = VertexInstanceUVs.GetNumChannels();
 		//Verify the src light map channel
 		if (BuildSettings->SrcLightmapIndex >= NumIndices)
 		{
@@ -83,7 +83,7 @@ void FMeshDescriptionHelper::SetupRenderMeshDescription(UObject* Owner, FMeshDes
 			}
 
 			//Add some unused UVChannel to the mesh description for the lightmapUVs
-			VertexInstanceUVs.SetNumIndices(BuildSettings->DstLightmapIndex + 1);
+			VertexInstanceUVs.SetNumChannels(BuildSettings->DstLightmapIndex + 1);
 			BuildSettings->DstLightmapIndex = NumIndices;
 		}
 		FStaticMeshOperations::CreateLightMapUVLayout(RenderMeshDescription,