This version of libpng has been patched to add APNG support.
libpng will (eventually) support this natively, as a departure from
the developers' initial stance on animation support, but the intended
release publication (using very similar code to our patches) is not
yet ready.

Once libpng 2.0.x (originally 1.8.x) is ready, our client-side patches
can likely be discarded and we can lean on upstream's direct support
from that point forward.
The apng.patch and arm.patch files are primarily for reference/as a
starting point for any libpng updates in the meantime.
