Enforce window rendering in sequence on llvmpipe.

When rendering multiple windows in parallel on llvmpipe we end up with
crashes deep inside llvmpipe as multiple threads seem to access
unprotected resources. Work around this bug by enforcing that scene
graph rendering happens on one window at a time.

Task-number: QTCREATORBUG-10666
Change-Id: I2f734e8f653b2a9b4108eb189280ab922581e2c0
Reviewed-by: Kai Koehne <kai.koehne@digia.com>