plugins: input-raw: Parse BTN_LEFT in MT mode

[dsemkowicz-tt]

Some devices, like eGalax USB HID touch panel, emulate a mouse device. In such case, BTN_LEFT instead of BTN_TOUCH event is read from evdev input. This case is already covered in ts_input_read(), but not in the ts_input_read_mt(). As ts_uinput tool always uses multitouch mode, such devices do not register touch events correctly.

Handle BTN_LEFT in the same way as BTN_TOUCH is handled to fix this problem. This will now be common with ts_input_read() implementation.