Update Class - Slider.lua

[mespotine]

localized internal variable _

I don't think, it had a negative impact, but nevertheless, I localized it, to be safe.

[jalovatt]

I just tried in the Lua command line and I think the _ = is actually unnecessary for performing a conditional operation like this - those might all be able to just do:

self.horz and gfx.circle(min, y + (h / 2), h / 2 - 1, 1, 1)
          or  gfx.circle(y + (h / 2), min, h / 2 - 1, 1, 1)