fixes #1157, #1255: cache parsed tiles in memory

src/mbgl/map/map.cpp

@@ -818,3 +818,26 @@ void Map::render() {
         triggerUpdate();
     }
 }
+
+void Map::setSourceTileCacheSize(size_t size) {
+    if (size != getSourceTileCacheSize()) {
+        invokeTask([=] {
+            sourceCacheSize = size;
+            if (!style) return;
+            for (const auto &source : style->sources) {
+                source->setCacheSize(sourceCacheSize);
+            }
+            env->performCleanup();
+        });
+    }
+}
+
+void Map::onLowMemory() {
+    invokeTask([=] {
+        if (!style) return;
+        for (const auto &source : style->sources) {
+            source->onLowMemory();
+        }
+        env->performCleanup();
+    });
+};