Bug 751553 - Linear precision doesn't display the image correctly

When converting and image with a color profileimage between linear and
gamma, create a new profile using the new API in GimpColorProfile,
convert the layers to that profile and tag the image with the new
profile.

If creating a new profile fails, convert to the right builtin profile
(linear rgb or sRGB from GimpColorProfile), but that code should be
considered a fallback that will be prevented from happening in the
convert dialog (at least the user will be informed).