introduce a global cache for color conversion transformation (it's internal to KoColorSpace but shared between instance), it replaced the old code that was specific to one instance, and didn't work when a color space is destroyed. On a side note, the new cache take into account the rendering intent.

The bug fix can't easily be applied to the stable branch, so next stable release won't include, but it's going to be in next major release.
BUG:151345

svn path=/trunk/koffice/; revision=743410