D10672 enabled taking screenshots of windows touching or going beyond
the border of the screen when compositing was disabled, i.e. KWin's DBus
screenshot mode was not in use. However, for some screen sizes Spectacle
would still only capture a null image. This could be observed in
particular for VMs with certain screen sizes.

The problem stems from using ints for dimensions, while
devicePixelRatio is a qReal. For setSize, QSize::operator*=() is
called, which uses qRound() and thus could also round up. This
resulted in dimensions passed to xcb_image_get() which were 1px too
large, resulting in the observed error.

The fix involves splitting setSize into setHeight and setWidth to
be able to control the rounding behaviour to always round down.
Specifying qFloor is not strictly necessary, but added for
explicitness.

Depends on D10672

• QT_SCALE_FACTOR=1.5 spectacle, turn off compositing via ⇧+Alt+F12.
• Move window to capture so it touches the bottom border of the screen, i.e. below the panel.
• Take screenshot in Active Window mode.
• Resize VM display vertically and repeat until a null image is captured.
• Apply patch and observe that now the correct image is captured.