Prevent background execution of zonal stats

This algorithm is NEVER safe to run in the background, because
it directly modifies a layer in place.

We should:
- make a variant of this algorithm which returns a new layer
AND
- refactor the QgsZonalStats code to create a separate map
of feature->stats, and then modify the existing Zonal Stats
algorithm to apply these changes in a post processing step

python/plugins/processing/algs/qgis/ZonalStatistics.py

@@ -32,6 +32,7 @@ from qgis.PyQt.QtGui import QIcon
 
 from qgis.analysis import QgsZonalStatistics
 from qgis.core import (QgsProcessing,
+                       QgsProcessingAlgorithm,
                        QgsProcessingParameterVectorLayer,
                        QgsProcessingParameterRasterLayer,
                        QgsProcessingParameterString,
@@ -61,6 +62,9 @@ class ZonalStatistics(QgisAlgorithm):
     def groupId(self):
         return 'rasteranalysis'
 
+    def flags(self):
+        return super().flags() | QgsProcessingAlgorithm.FlagNoThreading
+
     def __init__(self):
         super().__init__()
         self.bandNumber = None