D5438: [plugins/qpa] Do not replace a valid FBO with an invalid one

[Marco Martin]

mart accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R108 KWin

BRANCH
  qpa-fbo-valid

REVISION DETAIL
  https://phabricator.kde.org/D5438

To: graesslin, #plasma, #kwin, mart
Cc: plasma-devel, kwin, spstarr, progwolff, lesliezhai, ali-mohamed, hardening, 
jensreuterberg, abetts, sebas, apol