jrimage

Add initial srgb utility functions.

Merge branch 'master' of github.com:cbraley/jrimage

Merge in changes from other branch.

Saturating convert function is done and tested. It will fail a static_assert if you pass in mixed-sign arguments.

Working on saturating conversion, but mixed sign expressions are proving hard. I'm going to cop out now and enforce that both types have the same sign.