undefined behavior fix for pixel format macros
undefined behavior fix for pixel format macros

Though left shifting an unsigned char might seem like it would give
an unsigned int, it does not necessairly:

http://stackoverflow.com/questions/12557508/

This corrects the pixel format macros and lays them out a little more
clearly (though they probably do not belong in reb-c.h)

Caught by undefined behavior sanitizer.

kealist/ren-c,rgchris/ren-c,kealist/ren-c,giuliolunati/ren-c,draegtun/ren-c,codebybrett/ren-c,codebybrett/ren-c,draegtun/ren-c,codebybrett/ren-c,giuliolunati/ren-c,codebybrett/ren-c,rgchris/ren-c,rgchris/ren-c,giuliolunati/ren-c,draegtun/ren-c,hostilefork/rebol,hostilefork/rebol,rgchris/ren-c,hostilefork/rebol,giuliolunati/ren-c,kealist/ren-c,kealist/ren-c,hostilefork/rebol,draegtun/ren-c,kealist/ren-c,codebybrett/ren-c,draegtun/ren-c,rgchris/ren-c,kealist/ren-c,codebybrett/ren-c,draegtun/ren-c,hostilefork/rebol,hostilefork/rebol,giuliolunati/ren-c