Support more PDF filters

[schnaader]

Good explanation of filters, cascades and a list of available filters: http://blog.didierstevens.com/2008/05/19/pdf-stream-objects/

FlateDecode is already implemented

• Most important ones would be ASCII85Decode and LZWDecode
• Implementing cascades would be nice, but I'm not sure how common they are
• CCITTFaxDecode is very common, but probably hard to implement

[schnaader]

For ASCII85Decode, there's code in paq8pxd that can be used as a reference. It might be possible to modify/extend some of the Base64 routines in Precomp for it.