Add a slow first pass preset

Originally committed as revision 16477 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffpresets/libx264-slowfirstpass.ffpreset

+coder=1
+flags=+loop
+cmp=+chroma
+partitions=+parti8x8+parti4x4+partp8x8+partb8x8
+me_method=hex
+subq=6
+me_range=16
+g=250
+keyint_min=25
+sc_threshold=40
+i_qfactor=0.71
+b_strategy=2
+qcomp=0.6
+qmin=10
+qmax=51
+qdiff=4
+bf=4
+refs=1
+directpred=3
+trellis=0
+flags2=+bpyramid+wpred+dct8x8+fastpskip