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`

libcef/browser/osr/browser_platform_delegate_osr_mac.h

@@ -11,7 +11,9 @@
 class CefBrowserPlatformDelegateOsrMac : public CefBrowserPlatformDelegateOsr {
  public:
   explicit CefBrowserPlatformDelegateOsrMac(
-      std::unique_ptr<CefBrowserPlatformDelegateNative> native_delegate);
+      std::unique_ptr<CefBrowserPlatformDelegateNative> native_delegate,
+      bool use_shared_texture,
+      bool use_external_begin_frame);
 
   // CefBrowserPlatformDelegate methods:
   CefWindowHandle GetHostWindowHandle() const override;