GLVolume: split composite_id to object_id, volume_id and instance_id.

WIP: Added creation of GLVolumes for SLA supports.

src/libslic3r/SLAPrint.cpp

@@ -214,4 +214,14 @@ SLAPrintObject::SLAPrintObject(SLAPrint *print, ModelObject *model_object):
 
 SLAPrintObject::~SLAPrintObject() {}
 
+TriangleMesh SLAPrintObject::support_mesh() const
+{
+	return make_cube(10., 10., 10.);
+}
+
+TriangleMesh SLAPrintObject::pad_mesh() const
+{
+	return make_cube(10., 10., 10.);
+}
+
 } // namespace Slic3r