Add discard support to Vulkan

src/gpu/GrGpu.cpp

Index: src/gpu/GrGpu.cpp
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index 0a2592d38046085131bc9e3a41c96e3f6fa6e0a1..1397845f42de6d9bcf7d0ef931ee1e3672bae285 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -178,6 +178,14 @@ GrTexture* GrGpu::createTexture(const GrSurfaceDesc& origDesc, SkBudgeted budget
                 fStats.incTextureUploads();
             }
         }
+        // This is a current work around to get discards into newly created textures. Once we are in
+        // MDB world, we should remove this code a rely on the draw target having specified load
+        // operations.
+        if (isRT && texels.empty()) {
+            GrRenderTarget* rt = tex->asRenderTarget();
+            SkASSERT(rt);
+            rt->discard();
+        }
     }
     return tex;
 }