Extend getNonStdFontMap for non-embedded Impact fonts (bug 1365930)

According to https://en.wikipedia.org/wiki/Impact_(typeface) this font should be available on all current versions of Windows, and with the recently added font-substitution we should actually be able to render it correctly (at least on Windows).

src/core/standard_fonts.js

@@ -136,6 +136,7 @@ const getNonStdFontMap = getLookupTableFactory(function (t) {
   t["ComicSansMS-Bold"] = "Comic Sans MS-Bold";
   t["ComicSansMS-BoldItalic"] = "Comic Sans MS-BoldItalic";
   t["ComicSansMS-Italic"] = "Comic Sans MS-Italic";
+  t.Impact = "Helvetica";
   t["ItcSymbol-Bold"] = "Helvetica-Bold";
   t["ItcSymbol-BoldItalic"] = "Helvetica-BoldOblique";
   t["ItcSymbol-Book"] = "Helvetica";

test/pdfs/.gitignore

@@ -383,6 +383,7 @@
 !issue4650.pdf
 !issue6721_reduced.pdf
 !issue3025.pdf
+!bug1365930.pdf
 !french_diacritics.pdf
 !issue2099-1.pdf
 !issue3371.pdf

test/test_manifest.json

@@ -2329,6 +2329,12 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "bug1365930",
+       "file": "pdfs/bug1365930.pdf",
+       "md5": "0db06efce060d82da16a79e8bd762a1c",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "bug1337429",
        "file": "pdfs/bug1337429.pdf",
        "md5": "4e6e4dfdab884e9465bdce657b590028",