effects/screenshot: Prepare for versioned dbus interface
On Wayland, when the compositor sends a screenshot to the requesting app, it encodes the screenshot as a PNG image and sends the encoded data over the pipe. The requesting app (Spectacle) then needs to decode the data. The issue is that encoding PNG images is not cheap. This is the main reason why Spectacle is shown with a huge delay after you press the PrtScr key. In order to fix the latency issue, we need to transfer raw image data. Unfortunately, the current dbus api of the screenshot is too cluttered and the best option at the moment is to start with a clean slate. This change prepares the screenshot effect for versioned dbus interface. Most of existing dbus logic was moved out in a separate class. In order to schedule screen shots, the screenshot effect got some new API. QFuture<QImage> scheduleScreenShot(window, flags) QFuture<QImage> scheduleScreenShot(area, flags) QFuture<QImage> scheduleScreenShot(screen, flags) If a dbus interface needs to take a screenshot, it needs to call one of the overloaded scheduleScreenShot() functions. Every overload returns a QFuture object that can be used for querying the result. This change also introduces "sink" and "source" objects in the dbus api implementation to simplify handling of QFuture objects. Note that the QFutureInterface is undocumented, so if you use it, you do it on your own risk. However, since Qt 5.15 is frozen for non-commercial use and some other Plasma projects already use QFutureInterface, this is not a big concern. For what it's worth, in Qt 6, there's the QPromise class, which is equivalent to the QFutureInterface class. CCBUG: 433776 CCBUG: 430869
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