fftools/ffplay: do not write out of rdft visualization texture

fftools/ffplay: do not write out of rdft visualization texture

If the window is resized it was possible that xpos pointed outside the
visualization texture. By rearranging the overflow check we make sure this (and
a crash) does not happen.

We also don't have to use xleft for start position, as that is 0 anyways, and
if we ever want to take into account xleft then the texture should be
positioned accordingly when rendering.

Signed-off-by: Marton Balint <cus@passwd.hu>