[plugin/scale] fixed wrong increase and decrease

If the user manually set the desired scale by calling scale.set(1.60)
the scale_level was not set accordingly which meant that later doing
a Scale:Increase/Decrease command yielded incorrect scale amount.

data/plugins/scale.lua

@@ -32,6 +32,9 @@ local function set_scale(scale)
   local s = scale / current_scale
   current_scale = scale
 
+  -- we set scale_level in case this was called by user
+  scale_level = (scale - default_scale) / scale_steps
+
   if config.scale_mode == "ui" then
     SCALE = scale