Animated PNG (APNG) support for Imager.

```
use Imager;
use Imager::File::APNG;

my @frames = Imager->read_multi(file => "foo.png", type => "apng)
  or die Imager->errstr;

Imager->write_multi({
    file => "foo.png",
    type => "apng",
    apng_delay => 1/60,
  }, @frames)
  or die Imager->errstr;
```
