Keep a cache of the linear version of BablImage's needed for

Keep a cache of the linear version of BablImage's needed for
BablFormats around. This reduces the constant overhead for
conversions.
* babl/babl-classes.h: added .image_template field to BablFormat.
* babl/babl-format.c: (format_new): make .image_template NULL by
defualt.
* babl/babl-image.c: (babl_image_from_linear): use .image_template if
available instead of creating our own BablImage from scratch.
* babl/babl-memory.c: (babl_free): special case freeing of BablImage
and BablFormat to do the extra juggling needed for the image_template
cache.

svn path=/trunk/; revision=248