Support strokeAlpha/fillAlpha when creating a fallback appearance stream (issue 6810)

This fixes the colours, by respecting the strokeAlpha/fillAlpha-values, for a couple of Annotations in the PDF document from issue 13447.[1]

---
[1] Some of the annotations still won't render at all, when compared with Adobe Reader, but that could/should probably be handled separately.

test/pdfs/.gitignore

@@ -124,6 +124,7 @@
 !bug1068432.pdf
 !issue12295.pdf
 !bug1146106.pdf
+!issue13447.pdf
 !bug1245391_reduced.pdf
 !bug1252420.pdf
 !bug1513120_reduced.pdf

test/test_manifest.json

@@ -115,6 +115,19 @@
        "type": "eq",
        "about": "Causes cmap to be created with invalid glyph ids."
     },
+    {  "id": "issue13447-eq",
+       "file": "pdfs/issue13447.pdf",
+       "md5": "a729709950d69ec1cfc516bf559e995d",
+       "rounds": 1,
+       "type": "eq"
+    },
+    {  "id": "issue13447-annotations",
+       "file": "pdfs/issue13447.pdf",
+       "md5": "a729709950d69ec1cfc516bf559e995d",
+       "rounds": 1,
+       "type": "eq",
+       "annotations": true
+    },
     {  "id": "bug946506",
        "file": "pdfs/bug946506.pdf",
        "md5": "c28911b5c31bdc337c2ce404c5971cfc",