added checks on Decoder.objCurrent to handle case if it's nil.

object (o) and group (g) lines are optional in OBJ. the parser doesn't create a group if
these lines are not present, which breaks parsing of face and usemtl lines. I added checks
to create 'default' groups when first encountering a usemtl or face line if no group or
object was created.

loader/obj/obj.go

@@ -445,13 +445,19 @@ func (dec *Decoder) parseObject(fields []string) error {
 	if len(fields) < 1 {
 		return errors.New("Object line (o) with less than 2 fields")
 	}
+
+	dec.Objects = append(dec.Objects, makeObject(fields[0]))
+	dec.objCurrent = &dec.Objects[len(dec.Objects)-1]
+	return nil
+}
+
+// makes an Object with name.
+func makeObject(name string) Object {
 	var ob Object
-	ob.Name = fields[0]
+	ob.Name = name
 	ob.Faces = make([]Face, 0)
 	ob.materials = make([]string, 0)
-	dec.Objects = append(dec.Objects, ob)
-	dec.objCurrent = &dec.Objects[len(dec.Objects)-1]
-	return nil
+	return ob
 }
 
 // Parses a vertex position line
@@ -509,6 +515,14 @@ func (dec *Decoder) parseTex(fields []string) error {
 // f v1[/vt1][/vn1] v2[/vt2][/vn2] v3[/vt3][/vn3] ...
 func (dec *Decoder) parseFace(fields []string) error {
 
+	// NOTE(quillaja): this wasn't really part of the original issue-29
+	if dec.objCurrent == nil {
+		// if a face line is encountered before a group (g) or object (o),
+		// create a new "default" object. This 'handles' the case when
+		// a g or o line is not specified (allowed in OBJ format)
+		dec.parseObject([]string{fmt.Sprintf("unnamed%d", dec.line)})
+	}
+
 	// If current object has no material, appends last material if defined
 	if len(dec.objCurrent.materials) == 0 && dec.matCurrent != nil {
 		dec.objCurrent.materials = append(dec.objCurrent.materials, dec.matCurrent.Name)
@@ -614,6 +628,11 @@ func (dec *Decoder) parseUsemtl(fields []string) error {
 		return dec.formatError("Usemtl with no fields")
 	}
 
+	// NOTE(quillaja): see similar nil test in parseFace()
+	if dec.objCurrent == nil {
+		dec.parseObject([]string{fmt.Sprintf("unnamed%d", dec.line)})
+	}
+
 	// Checks if this material has already been parsed
 	name := fields[0]
 	mat := dec.Materials[name]