This implementation supports only single-frame sprites with a single
child object. As far as I know, all Flash effects in AliceSoft games
meet this requirement.
The DefineSound tag in SWF has raw PCM data, so we create a WAV file in-
memory and pass it to audio_play_archive_data(). This is not strictly
necessary since libsndfile can handle raw PCM data, but keeps the audio
and mixer interfaces simple.
This SWF parser is far from complete, in particular it only supports
simple bitmap-filled rectangular shapes, but it covers all the features
used in Daiteikoku, Rance Quest, Pastel Chime 3 and Dorapeko.
Flash files are stored in archives in AFF format, which is an encrypted
SWF wrapper. The decryption code was adapted from GARbro.