Using -vf eq and changing brightness, contrast, etc. using key bindings
with e.g. "add brightness 1" didn't work well: with step width 1, the
property gets easily "stuck". This is a rounding problem: e.g. setting
gamma to 3 would actually make it report that gamma is set to 2, so
the "add" command will obviously never reach 3 with a step width of 1.
Fix this by storing the parameters as integers.
This was broken in cac7702. This commit effectively changed these
properties to use the value as reported by vf_eq, instead of the
previously set value for the "add" command. This was more robust,
but not very correct either, so we keep the new behavior and make
vf_eq report its parameters more accurately.
d69b39512d