Add discard support to Vulkan

src/gpu/vk/GrVkRenderTarget.h

Index: src/gpu/vk/GrVkRenderTarget.h
diff --git a/src/gpu/vk/GrVkRenderTarget.h b/src/gpu/vk/GrVkRenderTarget.h
index 01ebf269df422ccd1b745b4e2a9275297c4b50b5..9628e3bdce8878a82ba5d599232bbae4ed5eb9ea 100644
--- a/src/gpu/vk/GrVkRenderTarget.h
+++ b/src/gpu/vk/GrVkRenderTarget.h
@@ -48,6 +48,7 @@ public:
         }
         return nullptr;
     }
+    GrVkImage* msaaImage() { return fMSAAImage; }
     const GrVkImageView* resolveAttachmentView() const { return fResolveAttachmentView; }
     const GrVkResource* stencilImageResource() const;
     const GrVkImageView* stencilAttachmentView() const;