Add discard support to Vulkan

src/gpu/GrGpu.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGpu.h"
 
@@ skipping 160 matching lines @@
     if (tex) {
         if (!caps->reuseScratchTextures() && !isRT) {
             tex->resourcePriv().removeScratchKey();
         }
         fStats.incTextureCreates();
         if (!texels.empty()) {
             if (texels[0].fPixels) {
                 fStats.incTextureUploads();
             }
         }
+        if (isRT && texels.empty()) {
+            GrRenderTarget* rt = tex->asRenderTarget();
+            SkASSERT(rt);
+            rt->discard();
+        }
     }
     return tex;
 }
 
 GrTexture* GrGpu::wrapBackendTexture(const GrBackendTextureDesc& desc, GrWrapOwnership ownership) {
     this->handleDirtyContext();
     if (!this->caps()->isConfigTexturable(desc.fConfig)) {
         return nullptr;
     }
     if ((desc.fFlags & kRenderTarget_GrBackendTextureFlag) &&
@@ skipping 274 matching lines @@
         fMultisampleSpecsMap.push_back_n(n, (const MultisampleSpecs*) nullptr);
     }
     fMultisampleSpecsMap[effectiveKey] = &specs;
     if (effectiveSampleCnt != desc.fSampleCnt) {
         SkASSERT(surfDescKey < effectiveKey);
         fMultisampleSpecsMap[surfDescKey] = &specs;
     }
     return specs;
 }