Atari colors into _gtia.h; NMIRES/NMIST as union

Atari colors into _gtia.h; NMIRES/NMIST as union

Use a C "union" to give both read (NMIST) and write (NMIRES) labels
to their shared register in ANTIC. (h/t @groessler).

Consolodate duplicated color definitions (HUE_..., COLOR_... and TGI_COLOR_...;
and the "_gtia_mkcolor()" macro), found in both "atari.h" and "atari5200.h",
moving it to "_gtia.h", which they both share (and which makes the most sense).