[core] Put gfx backend scope guard to Renderer::reduceMemoryUse()

Author: pozdnyakov

src/mbgl/renderer/renderer.cpp

@@ -116,6 +116,7 @@ void Renderer::dumpDebugLogs() {
 }
 
 void Renderer::reduceMemoryUse() {
+    gfx::BackendScope guard { impl->backend };
     impl->reduceMemoryUse();
     impl->orchestrator.reduceMemoryUse();
 }