plugin XEP-0264: Convert image to RGB if there is transparency:

JPEG doesn't support transparency, which was resulting in exception if a preview of an
image with transparency was made. This is now checked, and the image is converted if
necessary.

libervia/backend/plugins/plugin_xep_0264.py

@@ -167,6 +167,11 @@ class XEP_0264(object):
         with self.host.common_cache.cache_data(
             PLUGIN_INFO[C.PI_IMPORT_NAME], uid, MIME_TYPE, max_age
         ) as f:
+            # XXX: JPEG is used and doesn't support transparency, we need to convert
+            #   convert to RGB. We may want to change that in the future if another format
+            #   is used.
+            if img.mode in ("RGBA", "LA", "P"):
+                img = img.convert("RGB")
             img.save(f, SAVE_FORMAT)
             if fix_orientation:
                 log.debug(f"fixed orientation for {f.name}")