Support rendering to a hardware GL/D3D texture/surface provided by the client

[cztomczak]

Currently only Windows (D3D11) is supported in upstream CEF (master v71+ as of this writing).

Run upstream cefclient with these flags:

cefclient --off-screen-rendering-enabled --shared-texture-enabled
(optional) --external-begin-frame-enabled --off-screen-frame-rate=60

New off-screen-rendering APIs available in upstream CEF:

• CefWindowInfo.shared_texture_enabled
• CefWindowInfo.external_begin_frame_enabled
• CefBrowserHost::SendExternalBeginFrame
• CefRenderHandler::OnAcceleratedPaint

When --shared-texture-enabled flag is passed then the --disable-gpu and --disable-gpu-compositing flags should not be applied anymore.

There is one known issue with shared textures and PDF rendering currently, https://bitbucket.org/chromiumembedded/cef/issues/2516/windows-osr-pdf-viewer-does-not-render

Related issue and commits in upstream:

• Issue 1006: https://bitbucket.org/chromiumembedded/cef/issues/1006/cef3-support-rendering-to-a-hardware-gl
• PR 158 contains many useful comments on the implementation and possible issues: https://bitbucket.org/chromiumembedded/cef/pull-requests/158/support-external-textures-in-osr-mode/
• Commit 1: https://bitbucket.org/chromiumembedded/cef/commits/713eebc
• Commit 2: https://bitbucket.org/chromiumembedded/cef/commits/1e6b870

[cztomczak]

Current CEF master disabled this feature temporarily due to upstream Chromium API changes.