Determine the Soldiers At War palette

This commit also takes the first step towards making it configurable;
override `internal/palettes.DefaultPaletteName` at build time to choose
one palette over another. It would be nice to set this at runtime!

internal/palettes/palettes.go

@@ -0,0 +1,21 @@
+package palettes
+
+import (
+	"image/color"
+	"sync"
+)
+
+// Override this to change the palette globally
+const DefaultPaletteName = "ChaosGate"
+
+var (
+	Transparent = color.RGBA{R: 0, G: 0, B: 0, A: 0}
+
+	Palettes = map[string]color.Palette{}
+
+	initPalettes = sync.Once{}
+)
+
+func DefaultPalette() color.Palette {
+	return Palettes[DefaultPaletteName]
+}