fix addalpha with scrgb images

Author: jcupitt

CHANGELOG.rst

@@ -5,6 +5,7 @@
 - improve generation of `enums.py`
 - add `stream.py` example
 - fix a missing reference issue with custom sources
+- fix `addalpha` with scrgb images [RiskoZoSlovenska]
 
 ## Version 2.2.2 (released 4 Jan 2023)
 

pyvips/vimage.py

@@ -1886,6 +1886,8 @@ def addalpha(self):
         """Add an alpha channel."""
         if self.interpretation == 'grey16' or self.interpretation == 'rgb16':
             max_alpha = 65535
+        elif self.interpretation == 'scrgb':
+            max_alpha = 1.0
         else:
             max_alpha = 255
         return self.bandjoin(max_alpha)