Add discard support to Vulkan

src/gpu/vk/GrVkRenderTarget.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef GrVkRenderTarget_DEFINED
 #define GrVkRenderTarget_DEFINED
@@ skipping 30 matching lines @@
     ~GrVkRenderTarget() override;
 
     const GrVkFramebuffer* framebuffer() const { return fFramebuffer; }
     const GrVkImageView* colorAttachmentView() const { return fColorAttachmentView; }
     const GrVkResource* msaaImageResource() const {
         if (fMSAAImage) {
             return fMSAAImage->fResource;
         }
         return nullptr;
     }
+    GrVkImage* msaaImage() { return fMSAAImage; }
     const GrVkImageView* resolveAttachmentView() const { return fResolveAttachmentView; }
     const GrVkResource* stencilImageResource() const;
     const GrVkImageView* stencilAttachmentView() const;
 
     const GrVkRenderPass* simpleRenderPass() const { return fCachedSimpleRenderPass; }
     GrVkResourceProvider::CompatibleRPHandle compatibleRenderPassHandle() const {
         return fCompatibleRPHandle;
     }
 
     // override of GrRenderTarget
@@ skipping 75 matching lines @@
     int                        fColorValuesPerPixel;
 
     // This is a cached pointer to a simple render pass. The render target should unref it
     // once it is done with it.
     const GrVkRenderPass*      fCachedSimpleRenderPass;
     // This is a handle to be used to quickly get compatible GrVkRenderPasses for this render target
     GrVkResourceProvider::CompatibleRPHandle fCompatibleRPHandle;
 };
 
 #endif