diff -r 000000000000 -r 6474c204b198 gfx/thebes/gfxCachedTempSurface.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/thebes/gfxCachedTempSurface.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef GFX_CACHED_TEMP_SURFACE_H
+#define GFX_CACHED_TEMP_SURFACE_H
+
+#include "gfxASurface.h"
+#include "nsExpirationTracker.h"
+#include "nsSize.h"
+
+class gfxContext;
+
+/**
+ * This class can be used to cache double-buffering back surfaces.
+ *
+ * Large resource allocations may have an overhead that can be avoided by
+ * caching.  Caching also alows the system to use history in deciding whether
+ * to manage the surfaces in video or system memory.
+ *
+ * However, because we don't want to set aside megabytes of unused resources
+ * unncessarily, these surfaces are released on a timer.
+ */
+
+class gfxCachedTempSurface {
+public:
+  /**
+   * Returns a context for a surface that can be efficiently copied to
+   * |aSimilarTo|.
+   *
+   * When |aContentType| has an alpha component, the surface will be cleared.
+   * For opaque surfaces, the initial surface contents are undefined.
+   * When |aContentType| differs in different invocations this is handled
+   * appropriately, creating a new surface if necessary.
+   * 
+   * Because the cached surface may have been created during a previous
+   * invocation, this will not be efficient if the new |aSimilarTo| has a
+   * different format, size, or gfxSurfaceType.
+   */
+  already_AddRefed<gfxContext> Get(gfxContentType aContentType,
+                                   const gfxRect& aRect,
+                                   gfxASurface* aSimilarTo);
+
+  void Expire() { mSurface = nullptr; }
+  nsExpirationState* GetExpirationState() { return &mExpirationState; }
+  ~gfxCachedTempSurface();
+
+  bool IsSurface(gfxASurface* aSurface) { return mSurface == aSurface; }
+
+private:
+  nsRefPtr<gfxASurface> mSurface;
+  gfxIntSize mSize;
+  nsExpirationState mExpirationState;
+  gfxSurfaceType mType;
+};
+
+#endif /* GFX_CACHED_TEMP_SURFACE_H */