common: Introduce class type identifier apis.

common: Introduce class type identifier apis.

This identifier is useful when user identify the instance type in runtime.

ThorVG basically don't prefer to dynamic_cast() nor typeid(),
it compiles with -fno-rtti option for the optimial size.

Here is an example for the simple usage.

if (paint->identifier() == Shape::identifier())
  auto shape = static_cast<Shape*>(paint);

@Issue: https://github.com/Samsung/thorvg/issues/693