change API contract: disallow zero-sized images or surfaces

include/core/SkSurface.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_DEFINED
 #define SkSurface_DEFINED
 
 #include "SkRefCnt.h"
 #include "SkImage.h"
 #include "SkSurfaceProps.h"
 
 class SkCanvas;
 class SkPaint;
 class GrContext;
 class GrRenderTarget;
 
 /**
  *  SkSurface represents the backend/results of drawing to a canvas. For raster
  *  drawing, the surface will be pixels, but (for example) when drawing into
  *  a PDF or Picture canvas, the surface stores the recorded commands.
  *
  *  To draw into a canvas, first create the appropriate type of Surface, and
  *  then request the canvas from the surface.
+ *
+ *  SkSurface always has non-zero dimensions. If there is a request for a new surface, and either
+ *  of the requested dimensions are zero, then NULL will be returned.
  */
 class SK_API SkSurface : public SkRefCnt {
 public:
     SK_DECLARE_INST_COUNT(SkSurface)
 
     /**
      *  Create a new surface, using the specified pixels/rowbytes as its
      *  backend.
      *
      *  If the requested surface cannot be created, or the request is not a
@@ skipping 194 matching lines @@
 private:
     const SkSurfaceProps fProps;
     const int            fWidth;
     const int            fHeight;
     uint32_t             fGenerationID;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif