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..d82a062c3b9bbbdb307ef292ecde68785b948e1a 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -178,6 +178,11 @@ GrTexture* GrGpu::createTexture(const GrSurfaceDesc& origDesc, SkBudgeted budget
                 fStats.incTextureUploads();
             }
         }
+        if (isRT && texels.empty()) {

[bsalomon, 2016/06/30 13:47:31]

I imagine that this is not going to be a long term way of the initial discard.
Should we have a comment here about removing this once we auto insert discards
post-MDB?

+            GrRenderTarget* rt = tex->asRenderTarget();
+            SkASSERT(rt);
+            rt->discard();
+        }
     }
     return tex;
 }