This exposes cogl_sub_texture_new() and cogl_is_sub_texture() as
experimental public API. Previously sub-textures were only exposed via
cogl_texture_new_from_sub_texture() so there wasn't a corresponding
CoglSubTexture type. A CoglSubTexture is a high-level texture defined as
a sub-region of some other parent texture. CoglSubTextures are high
level textures that implement the CoglMetaTexture interface which can
be used to manually handle texture repeating.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
