Fix drawing with touchscreen

Right now annotating with the touchscreen uses synthetized mouse events and it's fine enough until we want to support multi finger draw, but for some reason (getting back to a coherent state after "fake" mouse events) after lifting the finger another event arrives with source as touchpad as a mouse move with no buttons to the actual cursor position

we should always reject events with no buttons, which the current condition didn't otherwise it produces an extra line being drawn