Fix render stop and high CPU load due to timing issue

Text generation is connected to pixmap generation thread started signal. However the signal may have been emitted faster than the connect could took place, and because started is fired only once, the connected lambda never got executed.

generatePixmap tried to sync up with that never happening text generation anyway by means of scheduling itself. This lead to a infinite loop via a no more sleeping QEventLoop.

Fixed by moving the connect in front of starting the thread.

Probably fixes bugs 396137, 396087, 403643. But I can't tell for sure if I really reproduced the same root cause, or just something similar.