wxNviz: fix initializing alpha in texture

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@48656 15284696-431f-4ddb-bdfa-cd5b030d7da7

gui/wxpython/gui_modules/wxnviz.py

@@ -1883,7 +1883,8 @@ class Texture(object):
         self.textDict = textDict
         
         # alpha needs to be initialized
-        self.image.InitAlpha()
+        if not self.image.HasAlpha():
+            self.image.InitAlpha()
     
         # resize image to match 2^n
         self.Resize()