Qt over VNC: Unsupported screen format, client region not rendered anymore

Commit I93ccd3c74bfbb0bd94b352476e5fe58a94119e1f (Solution for bug QTBUG-25998) introduced a regression when running Qt applications over VNC. The warning "Unsupported screen format: depth: 16, red_mask: 3f, blue_mask: f800" is shown and the client region of windows is not rendered anymore.

If I add the previous

switch (depth) {
case 32: return QImage::Format_ARGB32_Premultiplied;
case 24: return QImage::Format_RGB32;
case 16: return QImage::Format_RGB16;

to the end of the method as a fallback, rendering works again.