[dev-context] lpdf-ini.lmt: lpdf.tosixteen(), wrong conversion to UTF-16BE

9 Feb 2021

Hello,

conversion to UTF-16BE PDF strings used for example in bookmarks / PDF
outlines is not right.

Take the following example:

```
\starttext
\setupinteraction[state=start]
\placebookmarks[section][number=no]

\section[bookmark=𝕄]

\stoptext
```

Produces: <FEFF0075DD44> for 𝕄 (U+1D544), instead of the correct
<FEFFD835DD44>.


The relevant function is `lpdf.tosixteen()` (from lpdf-ini.lmt), and its
`cache`. (Although the same function is also in lpdf-aux.lmt, and in
MkIV equivalents).

My proposal (also enclosed as a file attachment):

```

--- a/lpdf-ini.lmt
+++ b/lpdf-ini.lmt
@@ -178,7 +178,8 @@
         if v < 0x10000 then
             v = format("%04x",v)
         else
-            v = format("%04x%04x",rshift(v,10),v%1024+0xDC00)
+            v = v - 0x10000
+            v = format("%04x%04x",rshift(v,10)+0xD800,v%1024+0xDC00)
         end
         t[k] = v
         return v
```

(Note the similiarity to existing function `big()` in l-unicode.lua.)

I found this by chance, but I am not really a ConTeXt user, so I hope
didn't miss anything.

Regards,
Michal Vlasák

    