Fix deadlock when number of point cloud files loaded >= available thr…

…eads

This would cause multiple QgsTasks to be created, each of which
uses one thread -- then if the number of tasks matches or exceeds
the number of available threads, then the call to blockingMapped
in QgsPointCloudStatsCalculator will hang forever waiting for
a thread to be freed (which will never happen, since the running
tasks CANNOT complete as they are blocked)

Avoid this by releasing and reserving the current thread
from the QgsPointCloudStatsCalculationTask before performing
the blockingMapped function.

src/core/pointcloud/qgspointcloudstatscalculator.cpp

@@ -209,7 +209,9 @@ bool QgsPointCloudStatsCalculator::calculateStats( QgsFeedback *feedback, const
 
   feedback->setProgress( 0 );
 
+  QThreadPool::globalInstance()->releaseThread();
   QVector<QgsPointCloudStatistics> list = QtConcurrent::blockingMapped( nodes, StatsProcessor( mIndex.get(), mRequest, feedback, 100.0 / ( double )nodes.size() ) );
+  QThreadPool::globalInstance()->reserveThread();
 
   for ( QgsPointCloudStatistics &s : list )
   {