Add ability to select a plane when generating RD-curves.
Update rd_plot.sh to produce an image for each metric.
Fix clang compiler warning

Compiling with clang-500.2.79 on Mac OS X gives this warning:

src/mcenc.c:2804:53: warning: for loop has empty body [-Wempty-body]
       && dp_pred->states[0].prevsi >= 0; dp_pred--);
src/mcenc.c:2804:53: note: put the semicolon on a separate line to silence this
      warning

Ignore the advice on how to fix it, and use a while loop instead.

Signed-off-by: Nathan E. Egge <negge@dgql.org>
Update .gitignore

Signed-off-by: Nathan E. Egge <negge@dgql.org>
Update RD collect scripts to use encoder output only.