qtbase: input: tslib: support multitouch using ts_read_mt()

I could actually do this myself, but I'd like opinions on it before I spend time here:

recent versions of tslib have a new API for both multitouch and singletouch devices, ts_read_mt().

The upcoming version of tslib will provide a robust way for Qt's plugin to use this, while keeping the exact current usage with ts_read(), in case an old version of tslib is installed on the user's system.

Now, we could use that safely. The thing is, tslib also includes a userspace evdev driver for itself, that people have running and configure Qt to use "evdevtouch" input plugin, pointing to tslib's evdev device.

Would you see benefit in having an updated tslib plugin in Qt? How would you estimate performance improvements in that case?