Implemented a unittest for saving HDR PNG files

The patch also fixes three bugs in saving of PQ images:

1. There should be no direct conversions from p2020-pq to/from U16 color spaces. It might happen that color conversion system will choose this edge for converting p709-g10-f32->p2020-g10-u16-> ->p2020->pq-u16, which is not correct and loses all HDR data.

2. PNG converter should also handle a case when we just save a already converted p2020->pq-u16 image, which should just go in pass-through way.

3. Fix a misprint in KisPNGConverter::buildImage