vcl: MenuFloatingWindow::InitClipRegion() should not be virtual

When OutputDevice's ImplInitClipRegion got marked as virtual, the
compiler raised a warning about colliding method names in
MenuFloatingWindow, and this got fixed by making the method virtual. But
making it virtual just makes the warning go away, doesn't fix the real
problem, that OutputDevice's ImplInitClipRegion() is no longer called,
leading to strange missing text in the menus.

In the meantime also ImplInitClipRegion() got renamed to
InitClipRegion(). Fix the problem by renaming InitClipRegion() to
InitMenuClipRegion() in MenuFloatingWindow.

Regression from 95711f5b9e7b6a982d1762d37d5a38e0f40b86f9 (fdo#74702 Move
ImplInitClipRegion Window code out of OutputDevice, 2014-04-25).

Change-Id: Ibeb5e8e99bf4369b349ac44376a2c217786ef318