let the text drawing path take a string it will not write to

DrawTextToScreen takes STR16, a mutable wchar_t*, for a string it only
measures and prints. Quest Debug System.cpp picks its text with a conditional:

  DrawTextToScreen( gubFact[ usLoop1 ] ? L"True" : L"False", ... );

which has type const wchar_t*. clang-cl's MSVC compatibility lets a bare
string literal decay to wchar_t*, but not a const wchar_t* expression, so it
reports

  error: no matching function for call to 'DrawTextToScreen'
  note: candidate function not viable: 1st argument ('const wchar_t *')
  would lose const qualifier

The parameter is const-correct in every function DrawTextToScreen hands the
string on to, so the change follows the string down the call chain rather
than stopping at the top:

  DrawTextToScreen, ShadowText          Utils/WordWrap.h
  WFStringPixLength                     Utils/Font Control.h
  mprintf, StringPixLength,             sgp/Font.h
  VarFindFontCenterCoordinates,
  VarFindFontRightCoordinates
  WinFontStringPixLength                sgp/WinFont.h
  gprintfdirty                          TileEngine/Render Dirty.h

The varargs members of that list copy the format string into a local buffer
before doing anything with it, and the measuring ones walk it and return a
width; none of the nine assigns through the parameter. Where StringPixLength
casts the string to UINT16* to walk it, the cast and the local now carry the
const rather than dropping it.

Callers are unaffected: every one already passes something that converts to
const CHAR16*. Both configurations build with no new diagnostics, and the
parse sweep drops from 9 error sites in 7 files to 5 in 3.

To check the claim that nothing writes through these parameters, read the
nine bodies; each is short.

sgp/WinFont.cpp

@@ -461,7 +461,7 @@ void PrintWinFont( UINT32 uiDestBuf, INT32 iFont, INT32 x, INT32 y, STR16 pFontS
 
 }
 
-INT16 WinFontStringPixLength( STR16 string2, INT32 iFont )
+INT16 WinFontStringPixLength( const CHAR16* string2, INT32 iFont )
 {
 	HWINFONT *pWinFont;
 	pWinFont = GetWinFont( iFont );
@@ -469,7 +469,7 @@ INT16 WinFontStringPixLength( STR16 string2, INT32 iFont )
 	if (pWinFont == NULL) return(0);
 
 if(g_lang == i18n::Lang::zh) {
-	wchar_t *p=string2;
+	const wchar_t *p=string2;
     UINT32 size = 0;
 	while (*p!=0)
 	{