Access the bbox/background data correctly in the MeshShadingPattern class (issue 18816)

This appears to have regressed in PR 13808, since it removed the `matrix`-entry from array returned by the `MeshShading.prototype.getIR` method *without* also updating the indexes in the `MeshShadingPattern` constructor.

test/pdfs/.gitignore

@@ -94,6 +94,7 @@
 !issue8424.pdf
 !issue8480.pdf
 !bug1650302_reduced.pdf
+!issue18816.pdf
 !issue8570.pdf
 !issue8697.pdf
 !issue8702.pdf

test/test_manifest.json

@@ -10078,6 +10078,13 @@
     "annotations": true,
     "type": "eq"
   },
+  {
+    "id": "issue18816",
+    "file": "pdfs/issue18816.pdf",
+    "md5": "45f863f5b227f700ffea1210e4f80403",
+    "rounds": 1,
+    "type": "eq"
+  },
   {
     "id": "issue17794",
     "file": "pdfs/issue17794.pdf",