[plugins/hwcomposer] Use an RGBA_8888 format for rendering surface

Summary:
When sharing surface and config used by the compositor scene with
QtQuick's OpenGL context an RGBA surface makes Qt way more happy.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2232