Fix text spacing with vertical fonts (#6387)

According to the PDF spec 5.3.2, a positive value means in horizontal,
that the next glyph is further to the left (so narrower), and in
vertical that it is further down (so wider).
This change fixes the way PDF.js has interpreted the value.

test/pdfs/.gitignore

@@ -100,6 +100,7 @@
 !issue5747.pdf
 !issue6099.pdf
 !issue6336.pdf
+!issue6387.pdf
 !issue6410.pdf
 !gradientfill.pdf
 !bug903856.pdf

test/test_manifest.json

@@ -1574,6 +1574,19 @@
        "lastPage": 1,
        "type": "eq"
     },
+    {  "id": "issue6387-canvas",
+       "file": "pdfs/issue6387.pdf",
+       "md5": "08c39ac6d0aab1596e6e59793eaf3ee4",
+       "rounds": 1,
+       "type": "eq"
+    },
+    {  "id": "issue6387-text",
+       "file": "pdfs/issue6387.pdf",
+       "md5": "08c39ac6d0aab1596e6e59793eaf3ee4",
+       "rounds": 1,
+       "type": "text",
+       "about": "Note that the text layer seems to be off to the right."
+    },
     {  "id": "issue4890",
        "file": "pdfs/issue4890.pdf",
        "md5": "1666feb4cd26318c2bdbea6a175dce87",