format.

Make swscale-test only perform the test from the input to the output
format rather than perform all.

Also implement swscale-test-all.sh, for performing all the tests.

Improve flexibility of the swscale-test tool, this way is simpler to
perform only a subset of tests.

Originally committed as revision 30825 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale