Wire the sliders into the config file

Not yet the game itself. That's still TODO.

internal/config/config.go

@@ -68,3 +68,36 @@ func (c *Config) Save() error {
 func (c *Config) DataFile(path string) string {
 	return filepath.Join(c.DataDir, path)
 }
+
+func (o *Options) ResolutionIndex() int {
+	if o.XRes == 640 && o.YRes == 480 {
+		return 1
+	}
+
+	if o.XRes == 800 && o.YRes == 600 {
+		return 2
+	}
+
+	if o.XRes == 1024 && o.YRes == 768 {
+		return 3
+	}
+
+	return 4 // Magic value
+}
+
+func (o *Options) SetResolutionIndex(value int) {
+	switch value {
+	case 1:
+		o.XRes = 640
+		o.YRes = 480
+	case 2:
+		o.XRes = 800
+		o.YRes = 600
+	case 3:
+		o.XRes = 1024
+		o.YRes = 768
+	}
+
+	// If the value isn't recognised, silently ignore the request to avoid
+	// overwriting options the resolution slider doesn't know about
+}