According to libavfilter/scale.c, if the width and height are both
less than or equal to 0 then the input size is used for both
dimensions. It does not need to be -1. -1:-1 is the same as 0:0 which
is the same as -10:-42, etc.

if (w < 0 && h < 0)
    eval_w = eval_h = 0;

The documentation for the zscale filter has also been updated since the
behavior is identical.
Signed-off-by: Kevin Mark <kmark937@gmail.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>