Implement popup support for OSR

[magreenblatt]

Original report by Joe Lauer (Bitbucket: jlauer, GitHub: jlauer).

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Hey @magreenblatt,

In Java-Cef's onBeforePopup() method, the native code checks this first:

if (browser->GetHost()->IsWindowRenderingDisabled()) {
    // Cancel popups in off-screen rendering mode.
    return true;
}

Is there a reason why popups would be disabled if in OSR-mode? Happy to submit a PR fix if this is simply legacy/forgotten code.

[magreenblatt]

It just hasn't been implemented yet (creating a popup window that renders with OSR). A PR would be welcome, thanks :)

[magreenblatt]

• changed title from "Popups Are Cancelled for Off Screen Rendered Windows" to "Implement popup support for OSR"
• changed kind from "bug" to "enhancement"