Add Atari palette.

This is defined in swgrpha.c in the original sources and appears to
be the palette that would have been used in the Atari port. The
red/blue color scheme also conveniently matches the terminology used
in the original documentation where the planes are referred to as being
red and blue.

src/sdl/video.c

@@ -66,6 +66,11 @@ VideoPalette VideoPalettes[] = {
 	{"Gas Plasma",
 		{{0x7d, 0x1b, 0x02}, {0xd3, 0x41, 0x00},
 		 {0xa8, 0x2e, 0x01}, {0xfe, 0x54, 0x00}}},
+	// Palette from swgrapha.c in the original source, the colors
+	// that would have appeared in the Atari port.
+	{"Atari",
+		{{0x00, 0x00, 0x00}, {0x00, 0x77, 0xff},
+		 {0xff, 0x00, 0x00}, {0xff, 0xff, 0xff}}},
 };
 
 bool vid_fullscreen = false;