update igors icons, x/y contraint for xlate
upgrade bld_prep.sh debian libpng, add rectify timeline audio pref, rework maskgui layout yet again, igors icons, indexfile size in kb, mask_scale toggle imgs
mask xy scale, mask boundary only overlay, fix 8 char mask nm bug, rework maskgui layout, rework maskauto classes/bezier draw, fix descratch popupmenu size, upgrade ffmpeg to 4.1.4