touchpad: rotate the touch part of tablets

Tablets in left-handed mode are rotated, so we need to rotate the touchpad
part of them too. This doesn't affect all tablets though, some of them are
symmetrical and the left-handed mode merely changes the button order around
(some of the earlier Bamboos). So we rely on libwacom to tell us which device
must be rotated.

The rotation itself is done on the input coordinate itself as we get it. This
way any software buttons, palm zones, etc. are automatically handled by rest
of the code.

Fixes #274

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>