[core] use stale sprite data

This is a naïve implementation that essentially merges updated data into existing data. It will *not* remove icons from the stale sprite if they aren't present in the fresh sprite (we aren't tracking the source of a sprite, and the user could have changed it as well). Similarly, it will not update icons that have changed in dimension. This is a rare edge case and probably not worth implementing.

test/fixtures/stale/sprite.json

@@ -0,0 +1,10 @@
+{
+  "noise": {
+    "x": 0,
+    "y": 0,
+    "width": 32,
+    "height": 32,
+    "pixelRatio": 1,
+    "sdf": false
+  }
+}

test/fixtures/stale/style_and_sprite.json

@@ -0,0 +1,26 @@
+{
+  "version": 8,
+  "name": "Water",
+  "sources": {
+    "mapbox": {
+      "type": "vector",
+      "url": "asset://test/fixtures/stale/streets.json"
+    }
+  },
+  "sprite": "http://127.0.0.1:3000/stale/sprite",
+  "layers": [{
+    "id": "background",
+    "type": "background",
+    "paint": {
+      "background-color": "red"
+    }
+  }, {
+    "id": "water",
+    "type": "fill",
+    "source": "mapbox",
+    "source-layer": "water",
+    "paint": {
+      "fill-pattern": "noise"
+    }
+  }]
+}

test/storage/cache_stale.cpp

@@ -72,3 +72,31 @@ TEST_F(Storage, CacheStaleStyleAndTileJSON) {
 
     checkRendering(map, "stale_style_and_tilejson", 1000ms);
 }
+
+TEST_F(Storage, CacheStaleStyleAndSprite) {
+    HeadlessView view(display, 1);
+
+    auto cache = SQLiteCache::getShared(":memory:");
+
+    // Rig the cache with an expired stylesheet.
+    const std::string stylePath = "stale/style_and_sprite.json";
+    const Resource styleResource{ Resource::Kind::Style, prefix + "/" + stylePath };
+    cache->put(styleResource, expiredItem(stylePath));
+
+    // Rig the cache with an expired sprite JSON.
+    const std::string spritejsonPath = "stale/sprite.json";
+    const Resource spritejsonResource{ Resource::Kind::SpriteJSON, prefix + "/" + spritejsonPath };
+    cache->put(spritejsonResource, expiredItem(spritejsonPath));
+
+    // Rig the cache with an expired sprite JSON.
+    const std::string spriteimagePath = "stale/sprite.png";
+    const Resource spriteimageResource{ Resource::Kind::SpriteImage, prefix + "/" + spriteimagePath };
+    cache->put(spriteimageResource, expiredItem(spriteimagePath));
+
+    DefaultFileSource fileSource(":memory:", ".");
+
+    Map map(view, fileSource, MapMode::Still);
+    map.setStyleURL(styleResource.url);
+
+    checkRendering(map, "stale_style_and_sprite", 1000ms);
+}