Remove layer extent and statistic variables from processing contexts

These variables take a lot of time to calculate and cause lots of
lengthy hangs in processing. (Eg add some moderately large rasters
to a project, then try to run any processing algorithm and QGIS
will freeze).

The layer extent can already be used in expressions via the
layer_property function, which only evalutes the extent if required
and only for layers it is used for.

The band stats for raster layers should be moved to a
band_statistic function in core which behaves the same way.

python/plugins/processing/core/parameters.py

@@ -76,26 +76,6 @@ def _expressionContext():
         context.appendScope(QgsExpressionContextUtils.mapSettingsScope(iface.mapCanvas().mapSettings()))
 
     processingScope = QgsExpressionContextScope()
-    layers = dataobjects.getAllLayers()
-    for layer in layers:
-        name = layer.name()
-        processingScope.setVariable('%s_minx' % name, layer.extent().xMinimum())
-        processingScope.setVariable('%s_miny' % name, layer.extent().yMinimum())
-        processingScope.setVariable('%s_maxx' % name, layer.extent().xMaximum())
-        processingScope.setVariable('%s_maxy' % name, layer.extent().yMaximum())
-        if isinstance(layer, QgsRasterLayer):
-            cellsize = (layer.extent().xMaximum()
-                        - layer.extent().xMinimum()) / layer.width()
-            processingScope.setVariable('%s_cellsize' % name, cellsize)
-
-    layers = dataobjects.getRasterLayers()
-    for layer in layers:
-        for i in range(layer.bandCount()):
-            stats = layer.dataProvider().bandStatistics(i + 1)
-            processingScope.setVariable('%s_band%i_avg' % (name, i + 1), stats.mean)
-            processingScope.setVariable('%s_band%i_stddev' % (name, i + 1), stats.stdDev)
-            processingScope.setVariable('%s_band%i_min' % (name, i + 1), stats.minimumValue)
-            processingScope.setVariable('%s_band%i_max' % (name, i + 1), stats.maximumValue)
 
     extent = iface.mapCanvas().fullExtent()
     processingScope.setVariable('fullextent_minx', extent.xMinimum())