markFeatureWriter: Ignore contextual anchors in non-contextual lookups

Otherwise we end up with positioning statements that have duplicated
mark classes like this:

pos base a
   <anchor 200 200> mark @MC_top
   <anchor 100 200> mark @MC_top;

(one is the regular anchor, and the other is the contextual one). Which
makes no sense (feaLib shouldn’t probably allow the same mark class to
be used multiple times in the same statement).

Lib/ufo2ft/featureWriters/markFeatureWriter.py

@@ -649,6 +649,11 @@ def _makeMarkToBaseAttachments(self):
                     # skip '_1', '_2', etc. suffixed anchors for this lookup
                     # type; these will be are added in the mark2liga lookup
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 assert not anchor.isMark
                 baseMarks.append(anchor)
             if not baseMarks:
@@ -669,6 +674,11 @@ def _makeMarkToMarkAttachments(self):
                 # skip anchors for which no mark class is defined
                 if anchor.markClass is None or anchor.isMark:
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 if anchor.number is not None:
                     self.log.warning(
                         "invalid ligature anchor '%s' in mark glyph '%s'; " "skipped",
@@ -700,6 +710,11 @@ def _makeMarkToLigaAttachments(self):
                 if number is None:
                     # we handled these in the mark2base lookup
                     continue
+                if anchor.isContextual:
+                    # Skip contextual anchors. We don't support them in ufo2ft
+                    # and isContextual is always False, but subclasses may
+                    # handle them (e.g. glyphsLib’s MarkFeatureWriter).
+                    continue
                 # unnamed anchors with only a number suffix "_1", "_2", etc.
                 # are understood as the ligature component having <anchor NULL>
                 if not anchor.key:

tests/featureWriters/markFeatureWriter_test.py

@@ -1818,7 +1818,6 @@ def test_contextual_anchors_no_mark_feature(self, testufo):
 
             lookup mark2base {
                 pos base a
-                    <anchor 200 200> mark @MC_top
                     <anchor 100 200> mark @MC_top;
             } mark2base;