input/pointer: don't trigger pointer bindings for emulated input

Prior to this commit, a tablet device could trigger mouse button down
bindings if the pen was pressed on a surface that didn't bind tablet
handlers -- but it wouldn't if the surface did bind tablet handlers.

We should expose consistent behavior to users so that they don't have to
care about emulated vs. non-emulated input, so stop triggering bindings
for any non-pointer devices.

sway/input/cursor.c

@@ -270,7 +270,7 @@ static enum sway_input_idle_source idle_source_from_device(
 		return IDLE_SOURCE_SWITCH;
 	}
 
-	assert(false);
+	abort();
 }
 
 void cursor_handle_activity(struct sway_cursor *cursor,