add spng plugin

libspng ( 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:
6 jobs for !74 with martin/spng in 2 minutes and 36 seconds (queued for 2 seconds)
latest merge request