Use a black backdrop for Luminosity SMasks when /BC is missing

It fixes #21346.

src/core/evaluator.js

@@ -544,6 +544,11 @@ class PartialEvaluator {
       if (smask?.backdrop) {
         colorSpace ||= ColorSpaceUtils.rgb;
         smask.backdrop = colorSpace.getRgbHex(smask.backdrop, 0);
+      } else if (smask?.subtype === "Luminosity") {
+        // Per PDF spec 11.6.5.2: when /BC is missing the backdrop is the
+        // initial value of the group colour space, which is black for all
+        // standard colour spaces.
+        smask.backdrop = "#000000";
       }
 
       newOpList = new CheckedOperatorList();

test/pdfs/.gitignore

@@ -925,3 +925,4 @@
 !issue18548_reduced.pdf
 !issue_cff_unsigned_bbox.pdf
 !90ms_rksj_h_sample.pdf
+!issue21346.pdf

test/test_manifest.json

@@ -14327,5 +14327,12 @@
     "md5": "d2606e2c6cc9e679b8b88c2800c6e1a9",
     "rounds": 1,
     "type": "eq"
+  },
+  {
+    "id": "issue21346",
+    "file": "pdfs/issue21346.pdf",
+    "md5": "16b39d943d268ebfa99ee57f1f470f53",
+    "rounds": 1,
+    "type": "eq"
   }
 ]