Support multitouch devices without BTN_LEFT
Drag and click events on touch devices which have no mouse buttons (eg, drawing tablets) are interpreted from tap and double-tap events. We can't rely on double-tap events in case the user disables them in favour of draggy. Sending BTN_LEFT to the surrogate device doesn't seem to be picked up as a click, since the device doesn't have the BTN_LEFT capability, and the way to add the capability isn't working.
A workaround could be to find the first device that has a BTN_LEFT and use that to send the click events while still sending the movement events from the multitouch device.