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
Mutter (and libmutter users) are the only users of this version of
cogl, and will more or less only use the cogl-1.0, cogl-2.0 and cogl
experimental API variants, and having the possibility of having
different API versions of the same API depending on what file includes
it is error prone and confusing. Lets just remove the possibility of
having different versions of the same API.
https://bugzilla.gnome.org/show_bug.cgi?id=768977
