Add st_texture_cache_load_file_to_cogl_texture

Add st_texture_cache_load_file_to_cogl_texture

For StWidget we want the ability to load raw CoglHandle references
rather than having a big pile of actors backing StWidget.

Several bits of StWidget expect to be able to synchronously load
textures; this is a crutch to support that until we can cleanly
make this asynchronous.

https://bugzilla.gnome.org/show_bug.cgi?id=607500