Add a test for multichannel downsample.
Add a test for multichannel downsample.

rgkoo/libmv-blender,SoylentGraham/libmv,Nazg-Gul/libmv,rgkoo/libmv-blender,Nazg-Gul/libmv,petertsoi/libmv,SoylentGraham/libmv,libmv/libmv,keir/libmv,petertsoi/libmv,libmv/libmv,SoylentGraham/libmv,Nazg-Gul/libmv,rgkoo/libmv-blender,rgkoo/libmv-blender,keir/libmv,libmv/libmv,keir/libmv,SoylentGraham/libmv,rgkoo/libmv-blender,libmv/libmv,Nazg-Gul/libmv,Nazg-Gul/libmv,petertsoi/libmv,keir/libmv,petertsoi/libmv