Fixes for iOS / Ganesh (consistent fs/vs uni precision decl and don't use stencil-only fbo for clear)

Review URL: http://codereview.appspot.com/4850043/


git-svn-id: http://skia.googlecode.com/svn/trunk@2050 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/include/GrGpu.h b/gpu/include/GrGpu.h
index fff40fb..d3c6c9d 100644
--- a/gpu/include/GrGpu.h
+++ b/gpu/include/GrGpu.h
@@ -493,6 +493,8 @@
 
     // GrGpu subclass removes the clip from the stencil buffer
     virtual void clearStencilClip(const GrIRect& rect) = 0;
+    // clears the entire stencil buffer to 0
+    virtual void clearStencil() = 0;
 
 private:
     GrContext*                  fContext; // not reffed (context refs gpu)