offscreen-effect: Rename create_target to create_buffer

offscreen-effect: Rename create_target to create_buffer

                *** This is an API change ***

The create_target() virtual function should return a CoglHandle to a
texture; clutter_offscreen_effect_get_target(), instead, returns a
CoglMaterial to be painted in the implementation of the paint_target()
virtual function.

Instead of equating textures with materials, and confusing the user of
the API, we should mark the difference more prominently.

First of all, we should return a CoglMaterial* (now that we have that
as a public type) in get_target(); having handles all over the place
does not make it easier to distinguish the semantics of the virtual
functions.

Then we should rename create_target() to create_texture(), to make it
clear that what should be returned is a texture that is used as the
backing for the offscreen framebuffer.