vo_opengl: change gamma suboption to take a value

The previous version of the gamma suboption was pretty useless. It could
be used to disable delayed gamma enabling, which is a mechanism to avoid
having to adjust gamma in the shader by default.

Repurpose the suboption and allow setting an exact gamma value with it.
You can already override gamma with the --gamma option as well as the
gamma input property, but these use a weird curve to create the
impression of a linear perceived brightness change when changing the
value. This suboption now allows setting an exact gamma value.
command: format volume property as integer for OSD

The value range is 0-100, so fractional values don't make much sense.
But the underlying data type is probably float to avoid getting "stuck"
when doing small volume increments. So step this around and pretend it's
an integer just on display.
command: use the step size for "add volume" commands

The step argument for "add volume <step>" was ignored until now. Fix it.

There is one problem: by defualt, "add volume" should use the value set
with --volstep. This value is 3 by default. Since the default volue for
the step argument is always 1 (and we don't really want to make the
generic code more complicated by introducing custom step sizes), we
simply multiply the step argument with --volstep to keep it compatible.

The --volstep option should probably be just removed in the future.