Remove a tiny bit of unnecessary "rgba" parsing in the getRGB function

This obviously won't matter in practice, however it seems more "correct" to only extract the necessary number of color components rather than slicing off excess ones at the end.

src/display/display_utils.js

@@ -604,9 +604,8 @@ function getRGB(color) {
   if (color.startsWith("rgba(")) {
     return color
       .slice(/* "rgba(".length */ 5, -1) // Strip out "rgba(" and ")".
-      .split(",")
-      .map(x => parseInt(x))
-      .slice(0, 3);
+      .split(",", 3)
+      .map(x => parseInt(x));
   }
 
   warn(`Not a valid color format: "${color}"`);