Add names for 5.0 and 5.1 back speaker channel layouts.

Originally committed as revision 18459 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/audioconvert.c

@@ -110,7 +110,9 @@ static const struct {
     { "surround",    3,  CH_LAYOUT_SURROUND },
     { "quad",        4,  CH_LAYOUT_QUAD },
     { "5.0",         5,  CH_LAYOUT_5POINT0 },
+    { "5.0",         5,  CH_LAYOUT_5POINT0_BACK },
     { "5.1",         6,  CH_LAYOUT_5POINT1 },
+    { "5.1",         6,  CH_LAYOUT_5POINT1_BACK },
     { "5.1+downmix", 8,  CH_LAYOUT_5POINT1|CH_LAYOUT_STEREO_DOWNMIX, },
     { "7.1",         8,  CH_LAYOUT_7POINT1 },
     { "7.1(wide)",   8,  CH_LAYOUT_7POINT1_WIDE },