fix image painted

Fixes following bug:
The `painted()` method of `image_32` should return `true` when something was rendered on it. However it returns `false` always when no rule of the last style is rendered.
The problem also affects 2.3.x.

The problem occurs [here](https://github.com/mapnik/mapnik/blob/9d363ce7e75b2aca496c5b5a22e29fe6cbf85225/include/mapnik/feature_style_processor_impl.hpp#L710). See that `painted` is set to `false` when `was_painted` has default value in contrary to [version 2.2.x](https://github.com/mapnik/mapnik/blob/dc7b943873bc961125c5a3eb6172aa0c81ad7ea6/include/mapnik/feature_style_processor_impl.hpp#L610) where only `true` can be set.

C++ test included.