mm_io_c: open MODE_CREATE files for reading and writing on Windows, too

mkvmerge's output file is opened in MODE_CREATE. This mode is supposed
to create the file if it doesn't exist, to truncate it otherwise and to
open it for writing. The fix for #1498 requires reading from the file,
too, and therefore the file must be opened for reading, too.

On non-Windows systems MODE_CREATE was already translated into opening
for reading and writing.

src/common/mm_io_win.cpp

@@ -59,7 +59,7 @@ mm_file_io_c::mm_file_io_c(const std::string &path,
       disposition = OPEN_ALWAYS;
       break;
     case MODE_CREATE:
-      access_mode = GENERIC_WRITE;
+      access_mode = GENERIC_WRITE | GENERIC_READ;
       share_mode  = FILE_SHARE_READ;
       disposition = CREATE_ALWAYS;
       break;