Upgrade to Doxygen 1.8.3.1


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1005 632fc199-4ca6-4c93-a231-07263d6284db
Add note regarding the fact that 4:4:0 lacks full SIMD support;  Add an option for benchmarking 4:4:0 subsampling in tjbench.


git-svn-id: svn://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1009 632fc199-4ca6-4c93-a231-07263d6284db