composite: log when disabled via OpenGLIsUnsafe
After updating my Debian buster to bullseye, I noticed missing shadows / composite for KDE, which made overlapping windows hard to recognize.
.xsession-errors just had "kwin_core: Compositing is not possible". Using the suggested QT_LOGGING_RULES="kwin_*.debug=true" didn't provide any more clues; nothing I tried did. Eventually I ended reading the kwin source and found, that disabled composite via kwinrc :: [Compositing] :: OpenGLIsUnsafe was not reported.
IMHO all these disabling cases should use qCCritical(KWIN_CORE), like the "Compositing is not possible" message, but for the time being, this simply logs the OpenGLIsUnsafe reason, just like all the others.