mir: Handle pointer up/down events without a button mask set (i.e. touch events)
mir: Handle pointer up/down events without a button mask set (i.e. touch events)

The Mir backend was checking for button mask changes to generate the appropriate
GDK event. When Mir generates a touch event it has no button mask. In this case
we'll just generate a primary button event.

chergert/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,ahodesuka/gtk,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,chergert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,ahodesuka/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,Adamovskiy/gtk,Lyude/gtk-,chergert/gtk,davidgumberg/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,Lyude/gtk-,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,chergert/gtk,ahodesuka/gtk