Always clearing to opaque black makes it impossible to create
semi-transparent overlays with linuxfb. Instead, behave like other
platforms' backingstores: if the target image has an alpha channel,
clear to transparent instead and set the correct composition mode.

Task-number: QTBUG-52475
Change-Id: I2db4588e0112e200a3aa5eed49f806c37d7ca8b5
Reviewed-by: Louai Al-Khanji <louai.al-khanji@qt.io>