Filter output from copypng

Currently copypng is used to copy pngs into bundles. As part of this some compression is attempted. In the case that the image has already been run through this compression, for example with pre-built `.bundle` directories, it produces this note:

```
libpng warning: Input PNG is already optimized for iPhone OS. Copying source file to destination...
```

I'm surprised this is enough to get past the `-q` passed to pngcrush, but apparently it is.

I'd like to add some filtering to this output to ignore these messages since we have many of these images, and it's quite noisy. I'm wondering if there's a reasonable way to do that, similar to how it's [done for `ibtool`](https://github.com/bazelbuild/rules_apple/blob/master/tools/xctoolrunner/xctoolrunner.py) despite the fact that it goes through the xcrunwrapper from bazel core which doesn't have this same filtering logic.