Reduce unnecessary usage of Array.prototype.concat()

There are obviously cases where using `concat` makes perfect sense, since that method doesn't change any of the existing Arrays; see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

However, in a few cases throughout the code-base that's not an issue and using `concat` only leads to unnecessary intermediate allocations. With modern JavaScript we can thus replace those with a combination of `push` and spread-syntax, which wasn't originally possible when the code was written.

src/display/text_layer.js

@@ -547,10 +547,7 @@ function expandBoundsLTR(width, bounds) {
       }
     }
 
-    Array.prototype.splice.apply(
-      horizon,
-      [i, j - i + 1].concat(changedHorizon)
-    );
+    Array.prototype.splice.apply(horizon, [i, j - i + 1, ...changedHorizon]);
   }
 
   // Set new x2 for all unset boundaries.