vf_eq: fix behavior when changing parameters

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.
command: simplify video brightness/gamma/etc. handling a bit

We don't need to store the offsets of the options corresponding to the
properties, because the option-property bridge knows about this already.

The check against 1000 was for the case if e.g. the --brightness option
is not used. Always overwrite the option value instead, both when
querying and setting the property. (This is needed to make the settings
persistent even if vf_eq is used and the video chain is reinitialized.)

This commit assumes that VFCTRL_SET_EQUALIZER is always paired with
VFCTRL_GET_EQUALIZER (likewise for VOCTRL), which is the case.