Add API to enable the caller to have a custom method for allocating an
external texture. This will enable the possibility for mutter to
generate a texture from for example an EGLStream without having to add
support for that in Cogl.
https://bugzilla.gnome.org/show_bug.cgi?id=773629
In cogl use cogl-config.h and in clutter use clutter-build-config.h. We
can't use clutter-config.h in clutter because its already used and
installed.
https://bugzilla.gnome.org/show_bug.cgi?id=768976
