Handle 'deferred clear codes' when decoding nonstandard gifs

This patch handles the 'deferred clear codes' clarification noted on
the coversheet of http://www.w3.org/Graphics/GIF/spec-gif89a.txt

--
    DEFERRED CLEAR CODE IN LZW COMPRESSION

    There has been confusion about where clear codes can be found in the
    data stream.  As the specification says, they may appear at anytime.  There
    is not a requirement to send a clear code when the string table is full.

    It is the encoder's decision as to when the table should be cleared.  When
    the table is full, the encoder can chose to use the table as is, making no
    changes to it until the encoder chooses to clear it.  The encoder during
    this time sends out codes that are of the maximum Code Size.

    As we can see from the above, when the decoder's table is full, it must
    not change the table until a clear code is received.  The Code Size is that
    of the maximum Code Size.  Processing other than this is done normally.

    Because of a large base of decoders that do not handle the decompression in
    this manner, we ask developers of GIF encoding software to NOT implement
    this feature until at least January 1991 and later if they see that their
    particular market is not ready for it.  This will give developers of GIF
    decoding software time to implement this feature and to get it into the
    hands of their clients before the decoders start "breaking" on the new
    GIF's.  It is not required that encoders change their software to take
    advantage of the deferred clear code, but it is for decoders.
--

These show up fairly frequently in the wild; ImageMagick handles them
correctly while ImageIO throws an error and JAI and GifDecoder only load
the top portion of the frames. Ex:
http://24.media.tumblr.com/tumblr_mceuhx0qgj1r5hubco1_500.gif
http://uberhumor.com/wp-content/uploads/2012/09/Gyvdz2.gif
https://i.chzbgr.com/completestore/12/10/23/H0I7k2e8M069BDdrEDb5ww2.gif
Merge branch 'non-standard-gifs'
Merge branch 'master' of github.com:rtyley/animated-gif-lib-for-java