qimage_conversion.cpp may deadlock applications when no threads are available in qtrheadpool::globalInstance

We have an application running a bunch of threads using QtConcurrent.
Upgrading to Qt 5.15, we discovered the app would deadlock, especially on machines with few cores.
Debugging this uncovered that qimage_conversions.cpp expects to get a thread, but in our case we actually use all threads and of of the threads in turn calls the conversion functions implicitly.