As the first step into removing Cogl types that are covered by
Graphene, remove CoglEuler and replace it by graphene_euler_t.
This is a mostly straightforward replacement, except that the
naming conventions changed a bit. Cogl uses "heading" for the
Y axis, "pitch" for the X axis, and "roll" for the Z axis, and
graphene uses the axis themselves. That means the 1st and 2nd
arguments need to be swapped.
Also adapt the matrix stack to store a graphene_euler_t in the
rotation node -- that simplifies the code a bit as well.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/458
In plenty of places a non-static function was defined but didn't have
the corresponding declaration. Fix this by adding them, or alternatively
making them static.
