rationale: encodePNG is only used in tests + sample apps, but shouldn't actually be part of the core library. In #7828, we're removing the libpng dependency for Android, and using a libpng-free version of encodePNG allows us to run the core unit tests on Android without pulling in libpng just for the test suite. In a next step, we should remove encodePNG() from the image.hpp header, which is a public header.
