Loosen requirement that there be only one GrDrawTarget

src/gpu/GrRenderTarget.cpp

Index: src/gpu/GrRenderTarget.cpp
diff --git a/src/gpu/GrRenderTarget.cpp b/src/gpu/GrRenderTarget.cpp
index e81a9cfd8382700485dc6f98463d9301b81e6e79..ad56cf761fff6e62afc5380d2532d9c82af48708 100644
--- a/src/gpu/GrRenderTarget.cpp
+++ b/src/gpu/GrRenderTarget.cpp
@@ -11,6 +11,7 @@
 
 #include "GrContext.h"
 #include "GrDrawContext.h"
+#include "GrDrawTarget.h"
 #include "GrGpu.h"
 #include "GrRenderTargetPriv.h"
 #include "GrStencilAttachment.h"
@@ -56,16 +57,26 @@ void GrRenderTarget::overrideResolveRect(const SkIRect rect) {
 
 void GrRenderTarget::onRelease() {
     SkSafeSetNull(fStencilAttachment);
+    fLastDrawTarget = nullptr;
 
     INHERITED::onRelease();
 }
 
 void GrRenderTarget::onAbandon() {
     SkSafeSetNull(fStencilAttachment);
+    fLastDrawTarget = nullptr;
 
     INHERITED::onAbandon();
 }
 
+void GrRenderTarget::setLastDrawTarget(GrDrawTarget* dt) {
+    if (fLastDrawTarget) {
+        SkASSERT(fLastDrawTarget->isClosed());
+    }
+
+    fLastDrawTarget = dt;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 bool GrRenderTargetPriv::attachStencilAttachment(GrStencilAttachment* stencil) {