atlas-texture: use COGL_TEXTURE_DEFINE macro

It was an oversight when making the CoglAtlasTexture api public that we
continued to use the COGL_TEXTURE_INTERNAL_DEFINE macro. This updates
the code to now use COGL_TEXTURE_DEFINE which means the
cogl_is_atlas_texture() function will now be exported in the public api.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
Reviewed-by: Lionel Landwerlin <llandwerlin@gmail.com>