Clear stencil buffer before using it for drawing

src/gpu/GrGpu.cpp

Index: src/gpu/GrGpu.cpp
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index e14e8929b04bbc84de2fa52f14298befac2733c2..0402e754588205fcb25365a80542c780ac5723e0 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -223,7 +223,11 @@ GrRenderTarget* GrGpu::wrapBackendRenderTarget(const GrBackendRenderTargetDesc&
         return nullptr;
     }
     this->handleDirtyContext();
-    return this->onWrapBackendRenderTarget(desc, ownership);
+    auto res = this->onWrapBackendRenderTarget(desc, ownership);
+    if (desc.fStencilBits) {

[bsalomon, 2016/09/16 13:46:21]

Can we have a comment something to the effect of:

// Our algorithms that use the stencil buffer assume it is initially cleared.

+        this->clearStencil(res);
+    }
+    return res;
 }
 
 GrRenderTarget* GrGpu::wrapBackendTextureAsRenderTarget(const GrBackendTextureDesc& desc) {