Merge pull request #206 from Unity-Technologies/fix_button_down_issue

fix mouse button down issue

engine/src/lib/ui/window/pointer_data_packet_converter.cc

           PointerData synthesized_hover_event = pointer_data;
           synthesized_hover_event.change = PointerData::Change::kHover;
           synthesized_hover_event.synthesized = 1;
-          synthesized_hover_event.buttons = state.previous_buttons;
 
           UpdateDeltaAndState(synthesized_hover_event, state);
           converted_pointers.push_back(synthesized_hover_event);