Create new cogl-utils.[ch] and move a helper function from
MetaShadowFactory there as meta_create_texture_material(); this
allows us to create single-layer materials from different parts of
Mutter and have them share the same template material.
Also expose a function for creating a 1x1 texture of a given
color meta_create_color_texture_4ub().
https://bugzilla.gnome.org/show_bug.cgi?id=634833
