[css-color-hdr] Add missing signed power function

Author: svgeesus

css-color-hdr-1/rec2100-hlg.js

@@ -51,4 +51,9 @@ function hlg_decode(RGB) {
         }
         return ((Math.exp((val - c) / a) + b) / 12) * scale;
     });
+}
+
+function spow (base, exp) {
+    let sign = base < 0? -1 : 1;
+	return sign * (Math.abs(base) ** exp);
 }