cogl-texture-2d-sliced: Store the internal format not image format

cogl-texture-2d-sliced: Store the internal format not image format

The 'format' member of CoglTexture2DSliced is returned by
cogl_texture_get_format. All of the other backends return the internal
format of the GL texture in this case. However the sliced backend was
returning the format of the image data used to create the texture. It
doesn't make any sense to retain this information because it doesn't
necessarily indicate the format of the actual texture. This patch
changes it to store the internal format instead.