stage-window: Add scaling factor accessors

We'll need to set and get the scaling factor of a ClutterStage from its
StageWindow implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=705915
stage: Adjust drawing to include the window scaling factor

In order to transparently support high DPI density displays, we must
maintain all coordinates and sizes exactly as they are now — but draw
them on a surface that is scaled up by a certain factor. In order to
do that we have to change the viewport and initial transformation
matrix so that they are scaled up by the same factor.

https://bugzilla.gnome.org/show_bug.cgi?id=705915
x11: Apply the window scaling factor

On high DPI density displays we create surfaces with a size scaled up by
a certain factor. Even if the contents stay at the same relative size
and position, we need to compensate the scaling both when changing the
surface size, and when dealing with input.

https://bugzilla.gnome.org/show_bug.cgi?id=705915
cogl: Compensate for window scaling

The common stage window code that we share on Cogl-based backends should
also use the scaling factor.

https://bugzilla.gnome.org/show_bug.cgi?id=705915
evdev: Cache the regexp

Instead of recreating it for every new device, we can cache the GRegex
and reuse it.

https://bugzilla.gnome.org/show_bug.cgi?id=707901
evdev: Clean up debug and error messages

https://bugzilla.gnome.org/show_bug.cgi?id=707901
Documentation fixes
Release Clutter 1.15.94 (snapshot)
Post-release version bump to 1.15.95