pywwt/layers.py: honor user-supplied vmax/vmin and make sure settings are applied

The way that the ImageLayer constructor was structured, any user specification
of the vmax and vmin values would be overridden. Restructure to prefer them,
and also make sure that the settings are applied but not in a redundant way.

pywwt/layers.py

@@ -595,28 +595,28 @@ class ImageLayer(HasTraits):
 
         self._image_url = self.parent._serve_file(self._sanitized_image, extension='.fits')
 
-        # Because of the way the image loading works in WWT, we may end up with
-        # messages being applied out of order (see notes in image_layer_stretch
-        # in wwt_json_api.js)
+        # Default the image rendering parameters. Because of the way the image
+        # loading works in WWT, we may end up with messages being applied out
+        # of order (see notes in image_layer_stretch in wwt_json_api.js)
         self._stretch_version = 0
 
+        data = fits.getdata(self._sanitized_image)
+        self.vmin = np.nanpercentile(data, 0.5)
+        self.vmax = np.nanpercentile(data, 99.5)
+
         self._initialize_layer()
 
-        # Force defaults
-        self._on_trait_change({'name': 'opacity', 'new': self.opacity})
-
-        self.observe(self._on_trait_change, type='change')
-
         if any(key not in self.trait_names() for key in kwargs):
             raise KeyError('a key doesn\'t match any layer trait name')
 
         super(ImageLayer, self).__init__(**kwargs)
 
-        # Determine initial stretch parameters
-        data = fits.getdata(self._sanitized_image)
+        # Apply settings (that the user may have overridden) and track changes
+        # automagically going forward.
+        self._on_trait_change({'name': 'vmin', 'new': self.vmin})
+        self._on_trait_change({'name': 'opacity', 'new': self.opacity})
+        self.observe(self._on_trait_change, type='change')
 
-        self.vmin = np.nanpercentile(data, 0.5)
-        self.vmax = np.nanpercentile(data, 99.5)
 
     @validate('stretch')
     def _check_stretch(self, proposal):