The interpolation and spread modes of morph shape gradients were
not being properly parsed. Fixed this and cleaned up some of the
gradient parsing code.
Improves the error handling for the swf crate:
* Custom swf::error::Error type added to handle various errors
in SWF parsing.
* Invalid parsing of tags/AVM1 ops results in a Error::ParseError
that can include info about the underlying failure.
* Implement Display for these errors. Output descriptive
names for the tag/opcode when it fails to parse.
* Handle out of bounds reads in avm1::Reader::read_slice.
Previously this would panic, now it returns an io::Error.
Closes#85.
Mike Welsh