Improve off-screen rendering behavior (issue #1257).
- The |dirtyRects| argument to CefRenderHandler::OnPaint is now a single rectangle representing the union of all dirty rectangles since the last frame was generated. In most cases this rectangle will be smaller than the view (frame) size.
- cefclient: Use a ScopedGLContext class on all platforms to manage the current GL context and avoid GL state leaks.
- cefclient: Add debug checks in ClientOSRenderer for GL errors and fix some errors that were discovered.
- cefclient: Add a new `--show-update-rect` command-line flag that provides visualization of the update rectangle by drawing a red border around it for each frame.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk@1945 5089003a-bbd8-11dd-ad1f-f1f9622dbc98