This is required to be compatible to the latest version of aero[1]. While using the addon,
the `WorldMapButton` does not return a proper `:GetCenter()` for a short period of time. This
leads to `mx` and `my` being nil and breaking the calculation. Now the coordinates will just
be skipped as long as `mx` and `my` could not be determined via `:GetCenter()`.
[1] https://github.com/gashole/Aero
some addons like to provide a broken _G variable to
the global environment breaking every other addon that
relies on it if existing. With this commit, make ShaguTweaks
no longer re-use the global _G, even if existing.
shagu