sd_ass: accept RFC8081 font media types

IETF RFC8081 added the "font" top-level media type,
including font/ttf, font/otf, font/sfnt, and also
font/collection. These font formats are all supported
by mpv/libass but they are not accepted as valid
Matroska mime types. mpv can load them via file extension
and they work as expected, so files using the new types
should not trigger a warning from mpv.

sub/sd_ass.c

@@ -87,6 +87,10 @@ static const char *const font_mimetypes[] = {
     "application/vnd.ms-opentype",
     "application/x-font-ttf",
     "application/x-font", // probably incorrect
+    "font/collection",
+    "font/otf",
+    "font/sfnt",
+    "font/ttf",
     NULL
 };