support for true-color terminals

[Charles Strahan]

Hello,

A while back, there was an effort to add support for true-color
terminals (by which I mean terminals supporting 16777216 simultaneous
colors):

http://emacs.1067599.n5.nabble.com/RFC-Add-tty-True-Color-support-td299962.html

I've contacted Rüdiger, asking if he wouldn't mind if I pick up from
where he left off, and he said that was fine as he's been too busy to
work on this.

I've attached to this email the patch that I'm using, based on his work.
Any feedback would be greatly appreciated.

This is my first (attempted) contribution to Emacs, and I'm very new to
the code base, so there might be some silly mistakes that I'm unaware
of. If we can get this polished enough for inclusion, I'd be more than
happy to assign copyright.

This patch is based off of Emacs 24.5. If it would be easier to get this
merged into Emacs 25, I can rebase my changes and submit a new patch for
review.


Thanks,

Charles

emacs-24.5-tty-true-color.patch
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