/usr/include/synfig-1.0/synfig/rendering/software/ is in libsynfig-dev 1.2.1-0ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/synfig-1.0/synfig/rendering/software/function/
/usr/include/synfig-1.0/synfig/rendering/software/function/array.h
/usr/include/synfig-1.0/synfig/rendering/software/function/blur.h
/usr/include/synfig-1.0/synfig/rendering/software/function/blurtemplates.h
/usr/include/synfig-1.0/synfig/rendering/software/function/contour.h
/usr/include/synfig-1.0/synfig/rendering/software/function/fft.h
/usr/include/synfig-1.0/synfig/rendering/software/function/packedsurface.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizerblendsw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizerblursw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizercontoursw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizerlayersw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizermeshsw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizerpixelcolormatrixsw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizerpixelgammasw.h
/usr/include/synfig-1.0/synfig/rendering/software/optimizer/optimizersurfaceresamplesw.h
/usr/include/synfig-1.0/synfig/rendering/software/renderersafe.h
/usr/include/synfig-1.0/synfig/rendering/software/renderersw.h
/usr/include/synfig-1.0/synfig/rendering/software/surfacesw.h
/usr/include/synfig-1.0/synfig/rendering/software/surfaceswpacked.h
/usr/include/synfig-1.0/synfig/rendering/software/task/
/usr/include/synfig-1.0/synfig/rendering/software/task/taskblendsw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskblursw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskcontoursw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskexpandsurfacesw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/tasklayersw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskmeshsw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskpixelcolormatrixsw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/taskpixelgammasw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/tasksurfaceresamplesw.h
/usr/include/synfig-1.0/synfig/rendering/software/task/tasksw.h