optimize findLoadedParent

Performance optimization when rendering view with a lot of satellite tiles.
Measurements on Chrome (Version 78.0.3904.108 with CPU 4x slowdown in performance tab) on MacMini i7 3.2 Ghz.

`map.repaint = true` in http://localhost:9966/debug/satellite.html#12.5/38.888/-77.01866/0/60

Observed FPS count:
master: 19-20FPS
with this patch: 24.5-26 FPS

src/source/source_cache.js

@@ -368,16 +368,14 @@ class SourceCache extends Evented {
      */
     findLoadedParent(tileID: OverscaledTileID, minCoveringZoom: number): ?Tile {
         for (let z = tileID.overscaledZ - 1; z >= minCoveringZoom; z--) {
-            const parent = tileID.scaledTo(z);
-            if (!parent) return;
-            const id = String(parent.key);
-            const tile = this._tiles[id];
+            const parentKey = tileID.calculateScaledKey(z, 1);
+            const tile = this._tiles[parentKey];
             if (tile && tile.hasData()) {
                 return tile;
             }
-            if (this._cache.has(parent)) {
-                return this._cache.get(parent);
-            }
+            const parentWrappedKey = tileID.calculateScaledKey(z, 0);
+            const data = this._cache.data[parentWrappedKey];
+            if (data) return data[0].value;
         }
     }
 

src/source/tile_id.js

@@ -95,6 +95,16 @@ export class OverscaledTileID {
         }
     }
 
+    calculateScaledKey(targetZ: number, wrap: 0 | 1) {
+        assert(targetZ <= this.overscaledZ);
+        const zDifference = this.canonical.z - targetZ;
+        if (targetZ > this.canonical.z) {
+            return calculateKey(this.wrap * wrap, targetZ, this.canonical.x, this.canonical.y);
+        } else {
+            return calculateKey(this.wrap * wrap, targetZ, this.canonical.x >> zDifference, this.canonical.y >> zDifference);
+        }
+    }
+
     isChildOf(parent: OverscaledTileID) {
         if (parent.wrap !== this.wrap) {
             // We can't be a child if we're in a different world copy