lib: Fix missing BVH in some cases

I had two booleans for the same thing, so sometimes we started the
renderer without a top-level BVH.

src/lib/api/c-ray.c

@@ -330,7 +330,7 @@ cr_instance cr_instance_new(struct cr_scene *s_ext, cr_object object, enum cr_ob
 		default:
 			return -1;
 	}
-	scene->instances_dirty = true;
+	scene->top_level_dirty = true;
 	return instance_arr_add(&scene->instances, new);
 }
 
@@ -367,7 +367,7 @@ void cr_instance_transform(struct cr_scene *s_ext, cr_instance instance, float r
 	struct matrix4x4 mtx = mtx_convert(row_major);
 	i->composite.A = mat_mul(i->composite.A, mtx);
 	i->composite.Ainv = mat_invert(i->composite.A);
-	scene->instances_dirty = true;
+	scene->top_level_dirty = true;
 }
 
 bool cr_instance_bind_material_set(struct cr_scene *s_ext, cr_instance instance, cr_material_set set) {

src/lib/renderer/renderer.c

@@ -123,7 +123,7 @@ void renderer_start_interactive(struct renderer *r) {
 }
 
 void update_toplevel_bvh(struct world *s) {
-	if (!s->top_level_dirty) return;
+	if (!s->top_level_dirty && s->topLevel) return;
 	struct bvh *new = build_top_level_bvh(s->instances);
 	//!//!//!//!//!//!//!//!//!//!//!//!
 	thread_rwlock_wrlock(&s->bvh_lock);
@@ -480,7 +480,6 @@ struct renderer *renderer_new(void) {
 
 	// Move these elsewhere
 	r->scene = calloc(1, sizeof(*r->scene));
-	r->scene->instances_dirty = true;
 	r->scene->asset_path = stringCopy("./");
 	r->scene->storage.node_pool = newBlock(NULL, 1024);
 	r->scene->storage.node_table = newHashtable(compareNodes, &r->scene->storage.node_pool);