cogl-texture-2d-sliced: Use the converted bitmap when uploading

cogl-texture-2d-sliced: Use the converted bitmap when uploading

When uploading texture data the cogl-texture-2d-sliced backend was
using _cogl_texture_prepare_for_upload to create a bitmap suitable for
upload but then it was using the original bitmap instead of the new
bitmap for the data. This was causing any format conversions performed
by cogl_texture_prepare_for_upload to be ignored.

http://bugzilla.openedhand.com/show_bug.cgi?id=2059