OBJLoader: When a new o/g starts, inherit the currently parsed material. (#9013)

Inherited material is overwritten if 'usemtl' declaration is encountered after the object starts but prior to next object start.
Exception to this is when faces have been declared to the inherited material, to keep MultiMaterial in sync.

Fixes #8792 and #8871.

examples/js/loaders/OBJLoader.js

@@ -95,6 +95,8 @@ THREE.OBJLoader.prototype = {
 
 				}
 
+				var previousMaterial = ( this.object && typeof this.object.currentMaterial === 'function' ? this.object.currentMaterial() : undefined );
+
 				this.object = {
 					name : name || '',
 					fromDeclaration : ( fromDeclaration !== false ),
@@ -111,6 +113,14 @@ THREE.OBJLoader.prototype = {
 
 						var previous = this._finalize( false );
 
+						// New usemtl declaration overwrites an inherited material, except if faces were declared
+						// after the material, then it must be preserved for proper MultiMaterial continuation.
+						if ( previous && ( previous.inherited || previous.groupCount <= 0 ) ) {
+
+							this.materials.splice( previous.index, 1 );
+
+						}
+
 						var material = {
 							index      : this.materials.length,
 							name       : name || '',
@@ -118,7 +128,21 @@ THREE.OBJLoader.prototype = {
 							smooth     : ( previous !== undefined ? previous.smooth : this.smooth ),
 							groupStart : ( previous !== undefined ? previous.groupEnd : 0 ),
 							groupEnd   : -1,
-							groupCount : -1
+							groupCount : -1,
+							inherited  : false,
+
+							clone : function( index ) {
+								return {
+									index      : ( typeof index === 'number' ? index : this.index ),
+									name       : this.name,
+									mtllib     : this.mtllib,
+									smooth     : this.smooth,
+									groupStart : this.groupEnd,
+									groupEnd   : -1,
+									groupCount : -1,
+									inherited  : false
+								};
+							}
 						};
 
 						this.materials.push( material );
@@ -144,6 +168,7 @@ THREE.OBJLoader.prototype = {
 
 							lastMultiMaterial.groupEnd = this.geometry.vertices.length / 3;
 							lastMultiMaterial.groupCount = lastMultiMaterial.groupEnd - lastMultiMaterial.groupStart;
+							lastMultiMaterial.inherited = false;
 
 						}
 
@@ -160,6 +185,20 @@ THREE.OBJLoader.prototype = {
 					}
 				};
 
+				// Inherit previous objects material.
+				// Spec tells us that a declared material must be set to all objects until a new material is declared.
+				// If a usemtl declaration is encountered while this new object is being parsed, it will
+				// overwrite the inherited material. Exception being that there was already face declarations
+				// to the inherited material, then it will be preserved for proper MultiMaterial continuation.
+
+				if ( previousMaterial && previousMaterial.name && typeof previousMaterial.clone === "function" ) {
+
+					var declared = previousMaterial.clone( 0 );
+					declared.inherited = true;
+					this.object.materials.push( declared );
+
+				}
+
 				this.objects.push( this.object );
 
 			},