First patch for async renderengine.

1. return a std::future object from DrawLayers() instead to make it
asynchronously
2. retire drawFence value from drawLayers() parameter list.

Bug: 180657548
Test: android.hardware.graphics.composer@2.2-vts, libcompositionengine_test, libsurfaceflinger_unittest and
librenderengine_test pass

Change-Id: If63c11762d1cce7b053cc72aaeaed6dd7904877f
diff --git a/libs/renderengine/RenderEngine.cpp b/libs/renderengine/RenderEngine.cpp
index 0c5a851..2174df5 100644
--- a/libs/renderengine/RenderEngine.cpp
+++ b/libs/renderengine/RenderEngine.cpp
@@ -95,5 +95,16 @@
                         "output buffer not gpu writeable");
 }
 
+std::future<RenderEngineResult> RenderEngine::drawLayers(
+        const DisplaySettings& display, const std::vector<const LayerSettings*>& layers,
+        const std::shared_ptr<ExternalTexture>& buffer, const bool useFramebufferCache,
+        base::unique_fd&& bufferFence) {
+    const auto resultPromise = std::make_shared<std::promise<RenderEngineResult>>();
+    std::future<RenderEngineResult> resultFuture = resultPromise->get_future();
+    drawLayersInternal(std::move(resultPromise), display, layers, buffer, useFramebufferCache,
+                       std::move(bufferFence));
+    return resultFuture;
+}
+
 } // namespace renderengine
 } // namespace android