More cleanup in GIFImageReader
https://bugs.webkit.org/show_bug.cgi?id=111137

Reviewed by Stephen White.

Refactor GIFImageReaderReader with the following changes:
+ Separate GIFLZWContext for decoding states.
+ Replace unsigned char* with Vector<unsigned char>

There is no change in code behavior and just refactoring.

No new tests. This is covered by existing GIFImageReaderTest.
I also did a local testing on a 50k image corpus and showed no regression.

* platform/image-decoders/gif/GIFImageDecoder.cpp:
(WebCore::GIFImageDecoder::haveDecodedRow):
* platform/image-decoders/gif/GIFImageDecoder.h:
(GIFImageDecoder):
* platform/image-decoders/gif/GIFImageReader.cpp:
(GIFImageReader::outputRow):
(GIFImageReader::doLZW):
(GIFImageReader::decodeInternal):
(GIFImageReader::prepareLZWContext):
* platform/image-decoders/gif/GIFImageReader.h:
(GIFFrameContext):
(GIFFrameContext::GIFFrameContext):
(GIFFrameContext::~GIFFrameContext):
(GIFLZWContext):
(GIFLZWContext::GIFLZWContext):
(GIFImageReader):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144961 268f45cc-cd09-0410-ab3c-d52691b4dbfc