Gracefully handle negative Encoding values

Back in 2012 libass added a custom Encoding=-1 extension to enable
automatic BiDi base direction instead of the usual LTR default.
The assumption/hope presumably was that since all valid font encodings
are positive VSFilter would just gracefully ignore them. However, this
is unfortuantely not the case.
When passed to GDI the charSet is cast to a byte and will wrap around to
a valid value, severely restricting which fonts can be used.

Searching through a large pile of subtitles the only hits for negative
values in the Encoding field were from generally corrupted files.
It thus appears so far fortunately nobody used the extension in released
fiels (only transient conversion from other  formats during playback)
and also nobody is relying on the wraparound in VSFilter.

To minimise future problems treat all negative Encodings as the default
charset. This matches how libass does fontselection for -1 albeit it
doesn't reproduce the BiDi base direction.
It also conveniently carves out the [-2, INT_MIN] range for potential
future extensions common to VSFilters and libass, e.g. Encoding=-2
for a RTL base direction (which should be easier to implement in
VSFilter than auto base direction)

src/subtitles/RTS.cpp

@@ -2415,6 +2415,7 @@ bool CRenderedTextSubtitle::ParseSSATag( CSubtitle* sub, const AssTagList& assTa
         case CMD_fe:
             {
                 int n = wcstol(p, NULL, 10);
+                if (n < 0) n = DEFAULT_CHARSET;
                 style.charSet = !p.IsEmpty()
                                 ? n
                                 : org.charSet;

src/subtitles/STS.cpp

@@ -1978,6 +1978,7 @@ static bool OpenSubStationAlpha(CTextFile* file, CSimpleTextSubtitle& ret, int C
             {
                 CString StyleName;
                 int alpha;
+                int encoding;
                 CRect tmp_rect;
 
                 StyleName = GetStr(buff);
@@ -2002,9 +2003,11 @@ if(sver >= 4)   style->borderStyle = GetInt(buff);
 if(sver >= 6)   tmp_rect.bottom = GetInt(buff);
                 style->marginRect = tmp_rect;
 if(sver <= 4)   alpha = GetInt(buff);
-                style->charSet = GetInt(buff);
+                encoding = GetInt(buff);
 if(sver >= 6)   style->relativeTo = GetInt(buff);
 
+                // Map unsupported extension to the most permissive charSet
+                style->charSet = encoding < 0 ? DEFAULT_CHARSET : encoding;
 if(sver <= 4)   style->colors[2] = style->colors[3]; // style->colors[2] is used for drawing the outline
 if(sver <= 4)   alpha = max(min(alpha, 0xff), 0);
 if(sver <= 4)   {for(size_t i = 0; i < 3; i++) style->alpha[i] = alpha; style->alpha[3] = 0x80;}