[Coverity] QOpenGLEngineManager uninit'ed value

Coverity¹ complains that, in QOpenGLEngineShaderManager::useCorrectShaderProg(), requiredProgram.compositionFragShader might be uninit'ed in L841, when calling sharedShaders->findProgramInCache(requiredProgram).

Indeed, the switch (compositionMode) in L777...813 does not initialize that value in its default: case label (L811..812).

Either we need to assign some no-op value in that case, or make the qWarning() a qCritical(), like it's done in L770, e.g.

¹ CID 390894 (https://scan3.scan.coverity.com/#/project-view/28157/10113?selectedIssue=390894)