Fix an issue that prevented tjEncodeYUV2() and TJCompressor.encodeYUV() from working...

Fix an issue that prevented tjEncodeYUV2() and TJCompressor.encodeYUV() from working properly if the source image was very tiny.  Basically, jpeg_start_compress() was attempting to write the JPEG headers, which was overrunning the YUV buffer.  This patch removes the call to jpeg_start_compress() in tjEncodeYUV2() and replaces it with calls to the individual routines that are necessary to initialize the color converter and downsampler.  TJUnitTest has also been modified to test for this condition (the buffer size regression test now works in YUV mode.)

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.2.x@1121 632fc199-4ca6-4c93-a231-07263d6284db