Introduce SurfaceInterface::hasFrameCallbacks()

The new method can be used to check if the surface has a pending frame
callback.

CCBUG: 428499

src/server/surface_interface.cpp

@@ -422,6 +422,11 @@ void SurfaceInterface::frameRendered(quint32 msec)
     }
 }
 
+bool SurfaceInterface::hasFrameCallbacks() const
+{
+    return !d->current.frameCallbacks.isEmpty();
+}
+
 QMatrix4x4 SurfaceInterfacePrivate::buildSurfaceToBufferMatrix(const State *state)
 {
     // The order of transforms is reversed, i.e. the viewport transform is the first one.

src/server/surface_interface.h

@@ -159,6 +159,7 @@ public:
     QMatrix4x4 surfaceToBufferMatrix() const;
 
     void frameRendered(quint32 msec);
+    bool hasFrameCallbacks() const;
 
     QRegion damage() const;
     QRegion opaque() const;