Import/decode .flac

[astrange]

The attached FLAC import patch doesn't seem to work - profile shows QT constantly opening and closing XiphQT FLAC in import_with_idle.

I think this is because our extradata is formatted wrong.

Ways to fix this:

• fix #443

• refactor DescExt_XiphFLAC in such a way that it can be shared with FFAvi

• implement our own FLAC decoder with a different fourcc (non-Xiph FLAC?). Same for Vorbis if we think the Xiph Vorbis extradata format is unnecessarily complicated.

[astrange]

patch that doesn't ever finish importing