Stub out Map#_rerender in paint benchmarks

Fixes #5543. Internal rendering changes were causing one of the conditions on the call to _rerender inside _render to be true, whereas before it was apparently always false. _rerender caused an animation frame to be requested, but then the resulting frameId would be reset via the benchmark's own call to _render. The result was a backlog of animation frame requests, some of which completed after teardown.

bench/benchmarks/layers.js

@@ -13,6 +13,8 @@ class LayerBenchmark extends Benchmark {
             style: this.layerStyle
         }).then(map => {
             this.map = map;
+            // Stub out `_rerender`; we want to be the only trigger of `_render`.
+            this.map._rerender = () => {};
         });
     }
 

bench/benchmarks/paint.js

@@ -16,7 +16,13 @@ module.exports = class Paint extends Benchmark {
                 center: [-77.032194, 38.912753],
                 style: 'mapbox://styles/mapbox/streets-v9'
             });
-        })).then(maps => { this.maps = maps; });
+        })).then(maps => {
+            this.maps = maps;
+            for (const map of this.maps) {
+                // Stub out `_rerender`; we want to be the only trigger of `_render`.
+                map._rerender = () => {};
+            }
+        });
     }
 
     bench() {