Fix a bug in the conversion of rgba32->yuv420p. This resulted in garbage images

Fix a bug in the conversion of rgba32->yuv420p. This resulted in garbage images
when this conversion was used. I suspect that the same bug may be lurking in
other conversions. [The assumption was that the linesize was equal to the width
for both the source and destination images. This turns out not to be true
in some cases.]

Originally committed as revision 1556 to svn://svn.ffmpeg.org/ffmpeg/trunk