add spng plugin

libspng (https://libspng.org/) is a faster and secure PNG library, with a
nicer API than libpng.
For now only decoding is implemented in this plugin.

I have been using it standalone for a while, this is basically just
an import of this + small modifications to fit better in kimageformats:
    https://invent.kde.org/sandsmark/qspng
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