[cantarell-fonts] Instantiator: use source.font if not None, else load

[Nikolaus Waxweiler <nwaxweiler src gnome org>]

commit 464d7aa28d444163184f1909b14e3d9aa60b04f1
Author: Nikolaus Waxweiler <madigens gmail com>
Date:   Mon Apr 1 23:09:30 2019 +0100

    Instantiator: use source.font if not None, else load

 scripts/instantiator.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/scripts/instantiator.py b/scripts/instantiator.py
index 8abdde6d..1fb75846 100644
--- a/scripts/instantiator.py
+++ b/scripts/instantiator.py
@@ -66,10 +66,13 @@ class Instantiator:
 
         glyph_names: Set[str] = set()
         for source in designspace.sources:
-            if not Path(source.path).exists():
-                raise ValueError(f"Source at path '{source.path}' not found.")
-            source.font = ufoLib2.Font.open(source.path, lazy=False)
-            glyph_names.update(source.font.keys())
+            if source.font is not None:
+                glyph_names.update(source.font.keys())
+            else:
+                if not Path(source.path).exists():
+                    raise ValueError(f"Source at path '{source.path}' not found.")
+                source.font = ufoLib2.Font.open(source.path, lazy=False)
+                glyph_names.update(source.font.keys())
 
         # Construct Variators
         axis_bounds: Dict[str, Tuple[float, float, float]] = {}