composite: adjust terminal messages (prio + text)

This makes the compositingPossible() output more elaborative,
and adjusts their priorities to qCWarning.

I thought about dropping the "Compositing is not possible"
(qCCritical) message. It's not critical for X11 and Wayland
depends on OpenGL. OTOH it's from a different component. So in
the end I settled for qCWarning again; there is no real way
to know, if the platform will consider this a critical problem
(and eventually abort).

Then I saw X11StandalonePlatform::compositingNotPossibleReason,
which has more user friendly, QT Richtext / HTML encoded output,
but this seems overkill for the terminal; now I would like to
know, where this is actually used...

While at it report "manually" suspended compositing via qCInfo
instead of qCDebug.