Resolve the long standing "NOTE" comments about having the same code in
wxAnimationDecoder::CanRead(), wxImage::CanRead() and GetImageCount() by
extracting the common logic into a helper wxInputStreamPeeker class and
using it from all places.
This loses the possibility to log a debug message if rewinding the
stream fails, but this is probably not very valuable and the actual
error should be already logged by SeekI() itself when it fails on a
seekable stream.
No real changes.
28c3605f6b