Compensate for global scale factor when using xcb fb plugin

The screen geometry is reported in device independent pixels, so it has
to be multiplied by the devicePixelRatio to get the size of the underlying
framebuffer. Otherwise, only the top left of the screen will be captured.

This matches the behavior of QScreen::grabWindow(...), which also
returns a QPixmap of the given size scaled by devicePixelRatio.

BUG: 419814