wacom: Clear background on "target" actor in calibration UI

the actor's draw() method isn't guaranteed to get a pristine surface,
which may cause artifacts if the background is left untouched, so
clear the background each time the actor has to be drawn.

https://bugzilla.gnome.org/show_bug.cgi?id=719701
wacom: Don't use monitor geometry X/Y when calculating calibration

Calibration must be performed based on the device w/h ranges and the
screen size, so in which coordinates the monitor is virtually mapped
matters little on device coordinates. This fixes calibration of devices
attached to a monitor elsewhere than (0,0) in the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=719574
wacom: Don't leave stray timelines on the calibration UI

Keep the reference for the error/helper messages animations in
the CalibArea struct, so those are destroyed and not leaked, or
possibly crashing when running on already destroyed actors if
the dialog gets cancelled at the right time.

https://bugzilla.gnome.org/show_bug.cgi?id=719698
wacom: Protect calibrator UI to spawn animations multiple times

The window state may be updated more than once, so only initiate
the UI and animations when it's first called.

https://bugzilla.gnome.org/show_bug.cgi?id=719698
wacom: Fix first animation of calibration UI

The "target" was seen moving from 0,0 to the first calibration
point, so 1) avoid target relayouts when the window is still
being positioned and 2) start the "target" actor as hidden so
it isn't seen moving from anywhere when first shown.

https://bugzilla.gnome.org/show_bug.cgi?id=719698