Discard scratch render targets before reusing them.

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 89548ab5c956ee4fd066cef2774746ef8264de18..8084557c933ff236d3d804ac4212681d799042c5 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -457,6 +457,11 @@ GrTexture* GrContext::lockAndRefScratchTexture(const GrTextureDesc& inDesc, Scra
         // Ensure we have exclusive access to the texture so future 'find' calls don't return it
         resource = fTextureCache->find(key, GrResourceCache::kHide_OwnershipFlag);
         if (NULL != resource) {
+            // If the scratch texture is a render target, discard its contents.
+            GrRenderTarget* rt = static_cast<GrTexture*>(resource)->asRenderTarget();
+            if (NULL != rt) {
+                rt->discard();
+            }
             resource->ref();
             break;
         }