Linux Skia: add an option to build a canvas from a provided memory buffer

skia/ext/platform_canvas_linux.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKIA_EXT_PLATFORM_CANVAS_LINUX_H_
 #define SKIA_EXT_PLATFORM_CANVAS_LINUX_H_
 
+#include <unistd.h>
+
 #include "skia/ext/platform_device_linux.h"
 
 namespace skia {
 
 // This class is a specialization of the regular SkCanvas that is designed to
 // work with a gfx::PlatformDevice to manage platform-specific drawing. It
 // allows using both Skia operations and platform-specific operations.
 class PlatformCanvasLinux : public SkCanvas {
  public:
   // Set is_opaque if you are going to erase the bitmap and not use
   // tranparency: this will enable some optimizations.  The shared_section
   // parameter is passed to gfx::PlatformDevice::create.  See it for details.
   //
   // If you use the version with no arguments, you MUST call initialize()
   PlatformCanvasLinux();
   PlatformCanvasLinux(int width, int height, bool is_opaque);
+  // Construct a canvas from the given memory region. The memory is not cleared
+  // first. @data must be, at least, @height * StrideForWidth(@width) bytes.
+  PlatformCanvasLinux(int width, int height, bool is_opaque, uint8_t* data);
   virtual ~PlatformCanvasLinux();
 
   // For two-part init, call if you use the no-argument constructor above
   bool initialize(int width, int height, bool is_opaque);
 
   // Returns the platform device pointer of the topmost rect with a non-empty
   // clip. Both the windows and mac versions have an equivalent of this method;
   // a Linux version is added for compatibility.
   PlatformDeviceLinux& getTopPlatformDevice() const;
 
+  // Return the stride (length of a line in bytes) for the given width. Because
+  // we use 32-bits per pixel, this will be roughly 4*width. However, for
+  // alignment reasons we may wish to increase that.
+  static size_t StrideForWidth(unsigned width);
+
  protected:
   // Creates a device store for use by the canvas. We override this so that
   // the device is always our own so we know that we can use GDI operations
   // on it. Simply calls into createPlatformDevice().
   virtual SkDevice* createDevice(SkBitmap::Config, int width, int height,
                                  bool is_opaque, bool isForLayer);
 
   // Creates a device store for use by the canvas. By default, it creates a
   // BitmapPlatformDevice object. Can be overridden to change the object type.
   virtual SkDevice* createPlatformDevice(int width, int height, bool is_opaque);
 
   // Disallow copy and assign.
   PlatformCanvasLinux(const PlatformCanvasLinux&);
   PlatformCanvasLinux& operator=(const PlatformCanvasLinux&);
 };
 
 }  // namespace skia
 
 #endif  // SKIA_EXT_PLATFORM_CANVAS_LINUX_H_