Don't map glyphs to certain problematic Thai/Lao Unicode locations (issue 5994)

*This patch depends on PR 5990.*

According to https://dxr.mozilla.org/mozilla-central/source/gfx/harfbuzz/src/hb-ot-shape-fallback.cc#38, certain Thai/Lao characters are treated as special by the font shaping code in Firefox.
Further down in that file, https://dxr.mozilla.org/mozilla-central/source/gfx/harfbuzz/src/hb-ot-shape-fallback.cc#216, the vertical position of glyphs is modified, which should thus explain why some glyphs end up in the wrong position in the PDF file.

Fixes 5994.

test/pdfs/.gitignore

@@ -56,6 +56,7 @@
 !issue5734.pdf
 !issue4875.pdf
 !issue4881.pdf
+!issue5994.pdf
 !rotated.pdf
 !issue1249.pdf
 !issue1171.pdf

test/test_manifest.json

@@ -1081,6 +1081,13 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "issue5994",
+       "file": "pdfs/issue5994.pdf",
+       "md5": "6799733a39d29b3828d6628bf2c5c382",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "zerowidthline",
       "file": "pdfs/zerowidthline.pdf",
       "md5": "295d26e61a85635433f8e4b768953f60",