Add presets to use as constraints for H.264 Baseline and Main profiles

Originally committed as revision 16584 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffpresets/libx264-baseline.ffpreset

+coder=0
+bf=0
+flags2=-wpred-dct8x8

ffpresets/libx264-main.ffpreset

+flags2=-dct8x8