Add SkSpecialImage & SkSpecialSurface classes

src/core/SkSpecialSurface.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file
+ */
+
+#ifndef SkSpecialSurface_DEFINED
+#define SkSpecialSurface_DEFINED
+
+#include "SkRefCnt.h"
+#include "SkSurfaceProps.h"
+
+class GrContext;
+struct GrSurfaceDesc;
+class SkCanvas;
+struct SkImageInfo;
+class SkSpecialImage;
+
+/**
+ * SkSpecialSurface is a restricted form of SkSurface solely for internal use. It differs
+ * from SkSurface in that:
+ *     - it can be backed by GrTextures larger than [ fWidth, fHeight ]
+ *     - it can't be used for tiling
+ *     - it becomes inactive once a snapshot of it is taken (i.e., no copy-on-write)
+ *     - it has no generation ID
+ */
+class SkSpecialSurface : public SkRefCnt {
+public:
+    const SkSurfaceProps& props() const { return fProps; }
+    int width() const { return fWidth; }
+    int height() const { return fHeight; }
+
+    /**
+    *  Return a canvas that will draw into this surface. This will always
+    *  return the same canvas for a given surface, and is managed/owned by the
+    *  surface. 
+    *
+    *  The canvas will be invalid after 'newImageSnapshot' is called.
+    */
+    SkCanvas* getCanvas();
+
+    /**
+    *  Returns an image of the current state of the surface pixels up to this
+    *  point. The canvas returned by 'getCanvas' becomes invalidated by this
+    *  call and no more drawing to this surface is allowed.
+    */
+    SkSpecialImage* newImageSnapshot();
+
+    /**
+     *  Allocate a new GPU-backed SkSpecialSurface. If the requested surface cannot
+     *  be created, nullptr will be returned.
+     */
+    static SkSpecialSurface* New(GrContext*, const GrSurfaceDesc&, const SkSurfaceProps* = nullptr);

[reed1, 2016/01/14 13:52:55]

I suggest we make the names back-end specific, just for more clarity at the
call-sites:

e.g.
    NewGpu(...)
    NewRaster(...)

[robertphillips, 2016/01/14 19:58:55]

Done.

+
+    /**
+     *  Return a new CPU-backed surface, with the memory for the pixels automatically
+     *  allocated.
+     *
+     *  If the requested surface cannot be created, or the request is not a
+     *  supported configuration, nullptr will be returned.
+     */
+    static SkSpecialSurface* New(const SkImageInfo&, const SkSurfaceProps* = nullptr);
+
+protected:
+    SkSpecialSurface(int width, int height, const SkSurfaceProps*);
+
+private:
+    const SkSurfaceProps fProps;
+    const int            fWidth;
+    const int            fHeight;
+
+    typedef SkRefCnt INHERITED;
+};
+
+#endif