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`
Windows still uses OpenGL 1.1 to avoid the added complexity of linking
newer OpenGL APIs on that platform. MacOS has deprecated OpenGL and we
should eventually provide a Metal-based implementation on that platform.
- Windows: 10.0.17763.0 SDK is now required.
- Mac: 10.13 SDK is now required.
- Removed CefRequestContext::ResolveHostCached which is no longer supported by Chromium.
