extern: Implement support for line input in fast_obj

When the .OBJ file specifies polylines via `l` command, we now preserve
them as is. Because faces and lines can interleave, and the lines are a
comparatively rare feature, we encode these as a per-face attribute and
let the caller figure it out. When face_lines[f] is 1, instead of
treating the face as a polygon, applications should treat it as a line
strip, with each vertex being connected to the previous one.

To avoid the overhead during parsing, we lazily allocate the array when
encountering the first line, and keep them synchronized afterwards.

Author: zeux

extern/fast_obj.h

@@ -137,6 +137,7 @@ typedef struct
     unsigned int                face_count;
     unsigned int*               face_vertices;
     unsigned int*               face_materials;
+    unsigned char*              face_lines;
 
     /* Index data: one element for each face vertex */
     unsigned int                index_count;
@@ -734,7 +735,7 @@ const char* parse_normal(fastObjData* data, const char* ptr)
 
 
 static
-const char* parse_face(fastObjData* data, const char* ptr)
+const char* parse_face(fastObjData* data, const char* ptr, unsigned char line)
 {
     unsigned int count;
     fastObjIndex vn;
@@ -796,6 +797,16 @@ const char* parse_face(fastObjData* data, const char* ptr)
     array_push(data->mesh->face_vertices, count);
     array_push(data->mesh->face_materials, data->material);
 
+    if (line || data->mesh->face_lines)
+    {
+        /* when line info exists, ensure it uses aligned indexing with other face data */
+        size_t skipped = array_size(data->mesh->face_vertices) - array_size(data->mesh->face_lines);
+        while (--skipped > 0)
+            array_push(data->mesh->face_lines, 0);
+
+        array_push(data->mesh->face_lines, line);
+    }
+
     data->group.face_count++;
     data->object.face_count++;
 
@@ -1295,7 +1306,22 @@ void parse_buffer(fastObjData* data, const char* ptr, const char* end, const fas
             {
             case ' ':
             case '\t':
-                p = parse_face(data, p);
+                p = parse_face(data, p, 0);
+                break;
+
+            default:
+                p--; /* roll p++ back in case *p was a newline */
+            }
+            break;
+
+        case 'l':
+            p++;
+
+            switch (*p++)
+            {
+            case ' ':
+            case '\t':
+                p = parse_face(data, p, 1);
                 break;
 
             default:
@@ -1398,6 +1424,7 @@ void fast_obj_destroy(fastObjMesh* m)
     array_clean(m->colors);
     array_clean(m->face_vertices);
     array_clean(m->face_materials);
+    array_clean(m->face_lines);
     array_clean(m->indices);
     array_clean(m->objects);
     array_clean(m->groups);
@@ -1454,6 +1481,7 @@ fastObjMesh* fast_obj_read_with_callbacks(const char* path, const fastObjCallbac
     m->colors         = 0;
     m->face_vertices  = 0;
     m->face_materials = 0;
+    m->face_lines     = 0;
     m->indices        = 0;
     m->materials      = 0;
     m->textures       = 0;