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/cefclient.cpp

@@ -332,3 +332,7 @@ void RunDragDropTest(CefRefPtr<CefBrowser> browser) {
 void RunGeolocationTest(CefRefPtr<CefBrowser> browser) {
   browser->GetMainFrame()->LoadURL("http://html5demos.com/geo");
 }
+
+void RunTransparencyTest(CefRefPtr<CefBrowser> browser) {
+  browser->GetMainFrame()->LoadURL("http://tests/transparency");
+}