osr: Implement shared texture support (fixes #1006, fixes #2575)

Adds support for the OnAcceleratedPaint callback. Verified to work
on macOS and Windows. Linux support is present but not implemented
for cefclient, so it is not verified to work.

To test:
Run `cefclient --off-screen-rendering-enabled --shared-texture-enabled`

include/cef_render_handler.h

@@ -152,16 +152,25 @@ class CefRenderHandler : public virtual CefBaseRefCounted {
   /// Called when an element has been rendered to the shared texture handle.
   /// |type| indicates whether the element is the view or the popup widget.
   /// |dirtyRects| contains the set of rectangles in pixel coordinates that need
-  /// to be repainted. |shared_handle| is the handle for a D3D11 Texture2D that
-  /// can be accessed via ID3D11Device using the OpenSharedResource method. This
-  /// method is only called when CefWindowInfo::shared_texture_enabled is set to
-  /// true, and is currently only supported on Windows.
+  /// to be repainted. |info| contains the shared handle; on Windows it is a
+  /// HANDLE to a texture that can be opened with D3D11 OpenSharedResource, on
+  /// macOS it is an IOSurface pointer that can be opened with Metal or OpenGL,
+  /// and on Linux it contains several planes, each with an fd to the underlying
+  /// system native buffer.
+  ///
+  /// The underlying implementation uses a pool to deliver frames. As a result,
+  /// the handle may differ every frame depending on how many frames are
+  /// in-progress. The handle's resource cannot be cached and cannot be accessed
+  /// outside of this callback. It should be reopened each time this callback is
+  /// executed and the contents should be copied to a texture owned by the
+  /// client application. The contents of |info| will be released back to the
+  /// pool after this callback returns.
   ///
   /*--cef()--*/
   virtual void OnAcceleratedPaint(CefRefPtr<CefBrowser> browser,
                                   PaintElementType type,
                                   const RectList& dirtyRects,
-                                  void* shared_handle) {}
+                                  const CefAcceleratedPaintInfo& info) {}
 
   ///
   /// Called to retrieve the size of the touch handle for the specified