Split from ticket:1570#comment:3. For macos, see #1794.
We currently fallback to non-opengl rendering for windows with the "has-alpha" flag. This is generally fine since transparent windows aren't normally used for fast moving content (the type of content that benefits the most from opengl accelerated rendering).
Can't be done with GTK3: ticket:1925#comment:2.
See also #893
DwmEnableBlurBehindWindow: Enables the blur effect on a specified window.
attachment/ticket/1570/win32-opengl-alpha.patch
This may help with ticket:2539#comment:21.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1682