add intfstream_truncate for file streams

Will return 0 for non-file streams.  I didn't want to mess around with
memory_stream (which could in theory have a truncate impl that sets
size) because there were globals and stuff and I got nervous (also
truncate might *grow* a file if the new length is longer than the old
one and then I'd have to think about realloc, etc).

libretro-common/include/streams/interface_stream.h

@@ -88,6 +88,9 @@ int intfstream_getc(intfstream_internal_t *intf);
 int64_t intfstream_seek(intfstream_internal_t *intf,
       int64_t offset, int whence);
 
+int64_t intfstream_truncate(intfstream_internal_t *intf,
+      uint64_t len);
+
 void intfstream_rewind(intfstream_internal_t *intf);
 
 int64_t intfstream_tell(intfstream_internal_t *intf);

libretro-common/streams/interface_stream.c

@@ -309,6 +309,34 @@ int64_t intfstream_seek(
    return -1;
 }
 
+int64_t intfstream_truncate(intfstream_internal_t *intf, uint64_t len)
+{
+   if (!intf)
+      return 0;
+
+   switch (intf->type)
+   {
+      case INTFSTREAM_FILE:
+         return filestream_truncate(intf->file.fp, len);
+      case INTFSTREAM_MEMORY:
+         break;
+      case INTFSTREAM_CHD:
+#ifdef HAVE_CHD
+         break;
+#else
+        break;
+#endif
+      case INTFSTREAM_RZIP:
+#if defined(HAVE_ZLIB)
+         break;
+#else
+         break;
+#endif
+   }
+
+   return 0;
+}
+
 int64_t intfstream_read(intfstream_internal_t *intf, void *s, uint64_t len)
 {
    if (!intf)