One new property, and the existing accel profile gets extended to keep one
extra value. The new property libinput Accel Curve Points is a list of pairs
of points to be added to the acceleration curve.

libinput only supports adding points to the curve so we simply declare the
behavior as undefined when the curve is set multiple times. Also helps to
identify those that bother to read the man page before playing with random
driver values.

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