bugfixes and an enhancement for enums

I came across a few issues when using ffi-swig-generator on the headers for libavcodec, libavformat, libavutil and libswscale; included in this push are fixes for those issues.

I also added an enhancement for the enum code to generate a FFI enum.  The main problem with this enhancement is that it breaks backwards compatibility because it no longer generates the old constants.  I can put together another patch to generate both the constants and the enums if you would like.