Create an empty stub for Vulkan readback

Create a stub for Vulkan readback. This avoids crashing,
because OpenGL readback does not work for Vulkan.

Test: Ran calc and gmail apps with skiavk pipeline.
Change-Id: I11cddde0a1efae1aa549e7a47d01051ec975f470

libs/hwui/pipeline/skia/SkiaVulkanReadback.h

@@ -0,0 +1,44 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "Readback.h"
+
+namespace android {
+namespace uirenderer {
+namespace skiapipeline {
+
+class SkiaVulkanReadback : public Readback {
+public:
+    SkiaVulkanReadback(renderthread::RenderThread& thread) : Readback(thread) {}
+
+    virtual CopyResult copySurfaceInto(Surface& surface, const Rect& srcRect,
+            SkBitmap* bitmap) override {
+        //TODO: implement Vulkan readback.
+        return CopyResult::UnknownError;
+    }
+
+    virtual CopyResult copyGraphicBufferInto(GraphicBuffer* graphicBuffer,
+            SkBitmap* bitmap) override {
+        //TODO: implement Vulkan readback.
+        return CopyResult::UnknownError;
+    }
+};
+
+} /* namespace skiapipeline */
+} /* namespace uirenderer */
+} /* namespace android */

libs/hwui/renderthread/RenderThread.cpp

@@ -24,6 +24,7 @@
 #include "hwui/Bitmap.h"
 #include "pipeline/skia/SkiaOpenGLPipeline.h"
 #include "pipeline/skia/SkiaOpenGLReadback.h"
+#include "pipeline/skia/SkiaVulkanReadback.h"
 #include "pipeline/skia/SkiaVulkanPipeline.h"
 #include "renderstate/RenderState.h"
 #include "renderthread/OpenGLPipeline.h"
@@ -158,12 +159,11 @@ Readback& RenderThread::readback() {
                 mReadback = new OpenGLReadbackImpl(*this);
                 break;
             case RenderPipelineType::SkiaGL:
-            case RenderPipelineType::SkiaVulkan:
-                // It works to use the OpenGL pipeline for Vulkan but this is not
-                // ideal as it causes us to create an OpenGL context in addition
-                // to the Vulkan one.
                 mReadback = new skiapipeline::SkiaOpenGLReadback(*this);
                 break;
+            case RenderPipelineType::SkiaVulkan:
+                mReadback = new skiapipeline::SkiaVulkanReadback(*this);
+                break;
             default:
                 LOG_ALWAYS_FATAL("canvas context type %d not supported", (int32_t)renderType);
                 break;