Refactor and clean up slider labels drawing code.

Refactor and clean up slider labels drawing code.

This change fixes multiple g++ 4 warnings about unsafe expressions with
bit-wise operators (completely justified, for once, as the expressions were
absolutely incomprehensible and almost certainly incorrect too) and refactors
the determination of where to draw the slider labels in a single function
instead of triplicating it.

Also reformat to follow wxWidgets conventions and use more clear variable
names.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61719 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775