blender: fix parallel builds
mplayer2: switch to libav to fix build

CC @viric. If someone has a better suggestion, do it.
IMHO the main problem is the lack of maintenance
from the side of mplayer2, e.g. latest release >3 years ago.
mpv: major update, incl. build fix with new ffmpeg

CC @AndersonTorres. The real build fix is in previous minor updates,
but I see no reason not to have the major updates as well.
Revert "qt4: try enableParallelBuilding again"

This reverts commit e8dc6258a1b5b11846fecf4a00c56a308dd22527.

It's unfortunate, but there still are occurrences of the missing ui_qtgradientview.h
on Hydra: http://hydra.nixos.org/build/12625390/nixlog/1/tail-reload