o renamed "aspectratio" to "sampleaspect", and changed semantics to
refer to actual sample aspect ratio (rather than display aspect ratio).
o "y4m_aspect_*" constants have become a more accurate and complete set
of "y4m_sar_*" constants.
o Updated the few programs which used aspectratio.
-Cleaned up Y4M library useage in yuvfilters and yuvmedianfilter a little.
