backends/native: Swap BTN_STYLUS and BTN_STYLUS2
BTN_STYLUS is the lower one and traditionally (read: in X) maps to middle button (2), BTN_STYLUS2 is the upper one and traditionally maps to right button (3).
This is also what GTK does and our desktop actions too map MIDDLE to BTN_STYLUS and RIGHT to BTN_STYLUS2.
See also gtk!6168 (merged)