[RFC] kscreen and touchscreen rotation

[Sebastian Kügler]

Hi all,

One of the things we talked about during Akademy was better support for 
convertible hardware. I've played around a bit with my Thinkpad X1 Yoga
and screen rotation. I can of course rotate the screen "manually"
through XRandR (Wayland is another story on that altogether), but that
will screw up the touchscreen. Doing some research, I found out that:

- touchscreen and display are completely separate things, the system
  doesn't know they're sitting on top of each other
- They need to be rotated separately
- rotation in X goes through XInput2, on Wayland, I don't know

I'm working on some code that does the rotation from KScreen right now.
My idea is to add this to KScreen's API and allow the KScreen user to
also rotate the display.

On X, this happens async, and this bears the risk that input ends up on 
different coordinates during switching (display has already rotated, 
touchscreen still in process, or some race condition like that -- I
don't think we can really prevent that), on Wayland, we should be able
to prevent that from happening as we hopefully can make input rotation
atomic along with the rotation itself, the protocol and API of screen
management in Wayland allow that.

We probably will need to guess which display has a touchscreen
attached, but for some cases, I think we can make some pretty
reasonable guesses 
- one display, one touchscreen seems like a the most common case, and
is clear
- one touchscreen and multiple displays: touchscreen may be on the
internal display
The mapping heuristic is kept internally, and we can work on that
later, once we got the basics in place.

Architecture / API design: My idea right now is to add a new relatively
simple type class to the KScreen API, TouchScreen, that can be received
from an Output, something like

output->setRotation(Output::Left);
TouchscreenList *touchscreens = output->touchscreens();
for (auto touchscreen : touchscreens) {
	touchscreens.at(0)->setRotation(Output::Left);
}
// ... then setting the new config as usual through SetConfigOperation()

This would have to also be implemented in the specific backends in
KScreen.

It's just an idea for now, but I'd like to get some feedback about it
at this point.
-- 
sebas

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