Windows: Improvements to off-screen rendering support (issue #518).

- Implement support for transparency. Change the backing store to use Skia instead of GDI and to keep alpha values.
- Implement support for select popups. Requires a patch to Chromium (content_popups.patch).
- Implicitly disable accelerated compositing when using off-screen rendering.
- Introduce a new CefMouseEvent structure for representing mouse event information passed to CefBrowser methods.
- Merge cef_key_event_modifiers_t into cef_event_flags_t.
- Add a new argument to CefBrowser::Invalidate telling the browser whether to invalidate the select popup or the main view.
- Add a new cefclient "transparent-painting-enabled" command-line flag to test transparent off-screen rendering.
- Add a new cefclient "Transparency" test.
- Fix the cefclient off-screen rendering rotation effect to work even when the underlying web content is not updating.
- Add unit tests for transparent painting and select popups.

git-svn-id: https://chromiumembedded.googlecode.com/svn/trunk@979 5089003a-bbd8-11dd-ad1f-f1f9622dbc98

tests/cefclient/client_switches.cpp

@@ -58,6 +58,7 @@ const char kAcceledated2dCanvasDisabled[] = "accelerated-2d-canvas-disabled";
 const char kAcceleratedPluginsDisabled[] = "accelerated-plugins-disabled";
 const char kDeveloperToolsDisabled[] = "developer-tools-disabled";
 const char kOffScreenRenderingEnabled[] = "off-screen-rendering-enabled";
+const char kTransparentPaintingEnabled[] = "transparent-painting-enabled";
 
 // Other attributes.
 const char kProxyType[] = "proxy-type";