Investigate switching to to libspng

[libspng](https://libspng.org) is an alternative to libpng that is both (allegedly) higher performance and also a much simpler dependency to manage (just one .h and one .c file, easily "vendored").

We should try a rewrite of our png handling to use this alternative library, check performance and see if we like the simplified dependency situation.

Review OpenImageIO's existing PNG handling and dependency setup, then evaluate libspng against libpng for performance and dependency simplicity. Done means the rewrite has been tried and the performance and vendoring trade-offs are documented clearly enough to decide whether to adopt it.

Written by the indexing model from the issue text.