Support for legacy files

[Melirius]

I found out that library does not support legacy Lepton files:

thread 'main' panicked at src\structs\lepton_header.rs:206:29:
attempt to subtract with overflow

It looks like it is possible to get in single-threaded mode.

[mcroomp]

Can you give me a file to test on? All legacy files should work.

[Melirius]

This is the file of DB Lepton test suite https://github.com/dropbox/lepton/blob/master/images/gold-legacy.lep

[Melirius]

Effectively I cannot find out in the code an equivalent of https://github.com/dropbox/lepton/blob/master/src/lepton/vp8_decoder.cc#L337 onwards, that is reading legacy file header. Then we arrive to lepton_header.rs:206:29 with empty thread_handoff