[plot] StackView issue with colormap
There seems to be an issue with `StackView` colormap handling.

To reproduce:

```
import numpy
from silx import sx                                                     
w = sx.StackView()                                                      
w.show()                                                                
w.setColormap(colormap='viridis', autoscale=True)                       
w.setStack(numpy.arange(10**3).reshape(10, 10, 10))
```

and the colormap used is switching to the grey one....

Setting the colormap with: `w.setColormap(Colormap('viridis'))` or `w.setColormap(colormap='viridis')` works fine.

This looks to be due to setting `self.__autoscaleCmap` (https://github.com/silx-kit/silx/blob/master/silx/gui/plot/StackView.py#L834), which looks like not needed any longer and causing trouble.

Thanks to Alessandro for spotting it.

diff --git a/silx/gui/plot/StackView.py b/silx/gui/plot/StackView.py
index 904e526e3..7e4c389c2 100644
--- a/silx/gui/plot/StackView.py
+++ b/silx/gui/plot/StackView.py
@@ -542,7 +542,12 @@ class StackView(qt.QMainWindow):
         # This call to setColormap redefines the meaning of autoscale
         # for 3D volume: take global min/max rather than frame min/max
         if self.__autoscaleCmap:
-            self.setColormap(autoscale=True)
+            # note: there is no real autoscale in the stack widget, it is more
+            # like a hack computing stack min and max
+            colormap = self.getColormap()
+            _vmin, _vmax = colormap.getColormapRange(data=self._stack)
+            colormap.setVRange(_vmin, _vmax)
+            self.setColormap(colormap=colormap)
 
         # init plot
         self._plot.addImage(self.__transposed_view[0, :, :],
['silx/gui/plot/StackView.py']