Clear stencil buffer before using it for drawing

src/gpu/GrGpu.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGpu.h"
 
@@ skipping 205 matching lines @@
         return tex;
     }
 }
 
 GrRenderTarget* GrGpu::wrapBackendRenderTarget(const GrBackendRenderTargetDesc& desc,
                                                GrWrapOwnership ownership) {
     if (!this->caps()->isConfigRenderable(desc.fConfig, desc.fSampleCnt > 0)) {
         return nullptr;
     }
     this->handleDirtyContext();
-    return this->onWrapBackendRenderTarget(desc, ownership);
+    auto res = this->onWrapBackendRenderTarget(desc, ownership);
+    if (desc.fStencilBits) {
+        // Our algorithms that use the stencil buffer assume it is
+        // initially cleared.
+        this->clearStencil(res);
+    }
+    return res;
 }
 
 GrRenderTarget* GrGpu::wrapBackendTextureAsRenderTarget(const GrBackendTextureDesc& desc) {
     this->handleDirtyContext();
     if (!(desc.fFlags & kRenderTarget_GrBackendTextureFlag)) {
       return nullptr;
     }
     if (!this->caps()->isConfigRenderable(desc.fConfig, desc.fSampleCnt > 0)) {
         return nullptr;
     }
@@ skipping 252 matching lines @@
         // This doesn't have geometric meaning. We just need to define an ordering for std::map.
         if (a[i].x() != b[i].x()) {
             return a[i].x() < b[i].x();
         }
         if (a[i].y() != b[i].y()) {
             return a[i].y() < b[i].y();
         }
     }
     return false; // Equal.
 }