Geometry.fromBufferGeometry(): Support drawcalls/offsets. See #5864.

src/core/BufferGeometry.js

@@ -764,7 +764,8 @@ THREE.BufferGeometry.prototype = {
 
 		/* Move all attribute values to map to the new computed indices , also expand the vertice stack to match our new vertexPtr. */
 		this.reorderBuffers( sortedIndices, revVertexMap, vertexPtr );
-		this.offsets = offsets;
+		this.offsets = offsets; // TODO: Deprecate
+		this.drawcalls = offsets;
 
 		/*
 		var orderTime = Date.now();

src/core/Geometry.js

@@ -154,9 +154,33 @@ THREE.Geometry.prototype = {
 
 		if ( indices !== undefined ) {
 
-			for ( var i = 0; i < indices.length; i += 3 ) {
+			var drawcalls = geometry.drawcalls;
 
-				addFace( indices[ i ], indices[ i + 1 ], indices[ i + 2 ] );
+			if ( drawcalls.length > 0 ) {
+
+				for ( var i = 0; i < drawcalls.length; i ++ ) {
+
+					var drawcall = drawcalls[ i ];
+
+					var start = drawcall.start;
+					var count = drawcall.count;
+					var index = drawcall.index;
+
+					for ( var j = start, jl = start + count; j < jl; j += 3 ) {
+
+						addFace( index + indices[ j ], index + indices[ j + 1 ], index + indices[ j + 2 ] );
+
+					}
+
+				}
+
+			} else {
+
+				for ( var i = 0; i < indices.length; i += 3 ) {
+
+					addFace( indices[ i ], indices[ i + 1 ], indices[ i + 2 ] );
+
+				}
 
 			}