added J Cut demos

[melted] / demo / mlt_jcut

inigo \
clip1.dv \
out=199 \
-track \
-blank 59 \
clip2.dv \
in=100 \
out=239 \
-transition \
composite \
start=0,0:100%x100%:0 \
key[5]=0,0:100%x100%:100 \
end=0,0:100%x100%:100 \
in=60 \
out=199 \
a_track=0 \
b_track=1 \
$*